MSc Chemistry
Analytical Sciences
Master Thesis
Metal-based nanoparticles used in restoration in cultural heritage
by
12156043
Catherine Gilbert
June 2020
12 ECTS
Period 4-5 2020
Supervisor/Examiner:
Second reviewer:
Prof. Alina Astefanei Prof. Klaas Jan van den Berg
Faculteit der Natuurwetenschappen, Wiskunde en Informatica
Van 't Hoff Institute for Molecular Sciences
ii Abstract
The following thesis is a review of a number of metal-based nanoparticle treatments that have been used in cultural heritage conservation, including silver, Ca(OH)2, magnesium oxides, TiO2, ZnO, and a number of others. For each
type of treatment, the methods of synthesis, mechanism of action, and examples of treatments that have been designed are considered, including those that have already been applied in cultural heritage. A key finding from this research is the importance of tailoring a material for its intended application. There are many examples of the same treatment having varying levels of success depending on the substrate to which it is applied. Key parameters to consider when designing such treatments are: the extent of damage, material of the object, and the climate in which the object is kept.
iii Contents
1. Introduction ... 1
1.1. Considerations for different materials used in art objects ... 2
1.1.1. Stone ... 2
1.1.2. Wood and Paper ... 4
1.1.3. Textiles ... 5
1.2. Analysis Methods used in the discussed literature... 6
1.2.1. Methods for the characterisation of cultural heritage objects ... 6
1.2.2. Analytical methods to determine the effectiveness of a treatment ... 7
1.2.3. Characterisation of NP colloids used in cultural heritage treatments ... 8
1.3. NPs used in the field of cultural heritage ... 9
2. Silver-based NPs ... 10
2.1. Properties ... 10
2.2. Mechanism of action... 10
2.2.1 Antimicrobial action of silver ... 10
2.2.2. Surface Plasmon Resonance ... 11
2.3. Synthesis of silver NPs ... 11
2.4. Applications of the antimicrobial activity of silver NPs ... 11
2.4.1. Application of silver NPs to paper and textile-based cultural heritage ... 12
2.4.2. Application of silver NPs to stone and cultural heritage monuments ... 13
2.5. Applications using SERS to analyse cultural heritage objects ... 13
3. Calcium Hydroxide ... 16
3.1. Properties ... 16
3.2. Synthesis of Ca(OH)2 NPs ... 17
3.3. Mechanism of Action ... 17
3.3.1. Deacidifying action of Ca(OH)2 NPs ... 17
3.3.2. Calcification of Ca(OH)2 NPs in stone and mortars ... 17
3.4. Consolidation of stone with Ca(OH)2 NPs ... 18
3.4.1. New Ca(OH)2 NP treatments not yet applied to cultural heritage ... 19
3.4.2. Case studies: Applications of Nanolimes to Cultural Heritage buildings ... 21
3.5. Deacidification of paper objects using Ca(OH)2 NPs ... 25
4. Magnesium Oxides ... 27
4.1. Properties ... 27
4.2. Synthesis ... 27
4.3. Mechanism of Action ... 27
4.3.1. Deacidifying action of Mg-based NPs ... 27
4.3.2. Antimicrobial activity of Mg-based NPs ... 28
iv
4.5. Other applications of Mg-based NPs ... 30
4.5.1. Consolidation of stones and mortars ... 30
4.5.2. Antimicrobial properties of Mg-based NPs ... 30
5. Titanium Dioxide ... 31
5.1. Properties ... 31
5.2. Synthesis ... 31
5.3. Mechanism of photocatalysis ... 31
5.4. TiO2 NPs used to create a self-cleaning treatment ... 33
5.4.1. New TiO2 treatments not yet applied to cultural heritage ... 33
5.4.2. Long-term effectiveness of self-cleaning treatments ... 36
5.5. Other applications of TiO2 NPs ... 37
5.5.1. Incorporation into lime mortars ... 37
5.5.2. Antimicrobial activity of TiO2 NPs ... 37
6. Zinc Oxide ... 39 6.1. Properties ... 39 6.2. Synthesis ... 39 6.3. Mechanism of Action ... 39 6.3.1. Photocatalysis mechanism ... 39 6.3.2. UV-protection mechanism ... 39
6.4. ZnO NPs used for UV-protection of cultural heritage objects ... 40
6.5. Self-cleaning activity of ZnO NPs ... 41
6.6. Antimicrobial activity of ZnO NPs ... 41
7. Miscellaneous ... 44 7.1. Zeolites ... 44 7.2. Metal-Organic Frameworks ... 44 7.3. Other Metal-based NPs ... 45 7.3.1. Zirconium Oxide ... 45 7.3.2. Copper ... 45 7.3.3. Nano-metal Fluorides... 46 7.3.4. Zinc borate ... 46 7.3.5. Strontium Hydroxide ... 46
8. Conclusions and future perspectives ... 47
8.1. Considerations when choosing a NP treatment ... 47
1
Metal-based nanoparticles used in restoration in cultural heritage
1. Introduction
There is increasing urgency to find effective treatments for fast degrading cultural heritage. The acceleration in degradation stems from increasingly harsh conditions for historical monuments caused climate change, and due to previous treatments applied to objects that have since proven to be incompatible. With growing interest in finding solutions to these problems, attention has turned to the field of nanotechnology for answers. A relatively new field of research itself, nanotechnology, and the use of metal-based nanoparticles (NPs), is now an established asset in many industries such as cosmetics, surface technology and catalysis. Now, its use in the field of heritage conservation has become popular in the last 15 or so years because of the fact that the properties and characteristics of these NPs are now well known, meaning that knowledge can be drawn from other fields, reducing the amount of time and money that must be spent on research in the cultural heritage field. The innate attraction in the use of these inorganic NPs is that they are stable in the long term, are hoped to be innately more chemically compatible with heritage materials, especially those made of stone and plaster, and should have a longer period of activity, in comparison to currently popular organic treatments such as polymer resins.
The most ideal method of conservation is preventative conservation, which involves handling materials as little as possible and keeping them in environments that protect them against damage, mainly in an environment with low humidity and limited UV exposure. However, for artifacts that have not been kept in these conditions, or when it is not possible to keep these conditions (i.e. buildings), more invasive treatments are often required. A recent trend in conservation science is using NPs. The term NPs refers to particles with a diameter of less than 100nm. The size of NPs makes them useful for in two main ways. Firstly, NPs have different optical properties than their bulk materials, causing interactions with UV and visible light. Secondly, they have a far higher surface area to volume ratio than the equivalent bulk materials, so the chemical activity of a compound is enhanced.
Furthermore, metal-based NPs are a promising alternative to conventional organic-based treatments, as metal-based materials are inherently more stable, and therefore should have a longer period of activity, minimalizing handling of delicate objects. Additionally, the physico-chemical activity of inorganic NPs is inherently more similar to the (mostly) inorganic substrates to which they are applied than alternative organic polymer consolidants. Figure 1 shows a common example of the accelerated degradation that can occur when using incompatible treatments, where the application of a synthetic polymer to a wall painting has resulted in the acceleration of damage by salt crystallisation.1
It is hoped that this new generation of consolidation treatments will not cause such damage on cultural heritage materials.
2 NP technology has been developed outside of the cultural heritage field, for medical applications, self-cleaning coatings, and mechanical reinforcement of structures.2–4 Recently, however, this technology has been applied in many
area of cultural heritage. When choosing a NP treatment for a cultural heritage object, the compatibility of the materials used is important when considering the long-term effectiveness of a procedure.5 Commercial NP-containing
products intended for cultural heritage restoration include treatments using NPs alone, such as calcium hydroxide-containing NanoRestore, or products such as poly siloxane resins can combined with NPs.6 It must be emphasised,
however, that the novelty of the field severely limits the available literature in terms of the long-term behaviour of the treatments discussed, were possible, the long-term activity has been discussed, but currently it is rarely reported. This thesis will not place a large focus on the possible toxicity of various NPs, since there is limited systematic research into the effects or extent of environmental exposure. The NP treatments discussed in detail in this thesis are widely used in other industries as they are believed to have limited toxicity to humans. When considering using NPs, one must consider toxicity in relation to a few factors, (i) the possible methods of uptake (ingestion, inhalation, or transdermal), (ii) the probability of exposure, (iii) the length of time and concentration of exposure, and (iv) the difference between the properties of the bulk and nanomaterials.7 In order to control for some of these risks, several measures can be
employed, including ensuring strong adherence of the NPs to the substrate, and the use of personal protective equipment during application to avoid high exposure. Furthermore, NPs purchased commercially are generally provided as a colloid, where the particles are suspended in a solvent, this is mainly to avoid the accidental inhalation of NPs.
For each type of NPs discussed, the most common method of synthesis will be mentioned, however, there are many different methods that can be used to change the size, shape, and polymorphic structure of NPs. These methods include mechanochemical synthesis, the sol gel method, reduction, and subsequent precipitation, chemical vapor deposition, hydrothermal synthesis, and the use of surfactants and other chemical modifications to control the size, chemical properties and colloidal stability of the NPs.8
Before considering the different treatments with NPs used in cultural heritage conservation, we must first consider the chemistry of the degradation of art objects. The main dangers to cultural heritage objects are humidity or water damage, light degradation, microbial infestation, and degradation caused by dirt or acidic vapours. The treatment of objects, and the types damage observed, varies from material to material.
1.1. Considerations for different materials used in art objects 1.1.1. Stone
There are three commonly agreed upon methods for determining the degree of damage in cultural heritage buildings, the comprehensive analysis of samples collected from the field, tests in simulation chambers and accelerated aging experiments, and field exposure tests.9 Developing treatments for historical buildings is made easier by the fact that
the source of the rock used is often known, and so representative test samples can be taken from a quarry instead of the building. Damage to historical buildings is being accelerated by the more extreme weather caused by climate change, as well as pollutions in cities, meaning that the number of monuments in need of restoration is rapidly increasing. Research into the preservation of the mechanical strength of bricks and mortar is very common as this is an area of interest in the building industry.
The primary cause of damage to historical monuments is the presence of water in the porous microstructures of the stone. Highly porous stones, with a high surface roughness, such as limestone, are more susceptible to erosion since there is a greater surface area that can interact with the outside elements.10,11 Both the circulation and freezing of
water in these pores cause significant structural damage to stones, through erosion, the expansion of freezing water, and dissolution of minerals contained in the stone, which can cause white stains on the surface, visible in Figure 2(a).12,13 Prediction models can be used in order to predict the sorptivity of limestones, which in turn can predict the
future degradation patterns of the rock, without requiring large amounts of rock samples for testing.12 Furthermore,
the geographical location of the monument can give indications of the future degradation pathways, for example increased salt crystallisation damage to stones in marine areas, or a reduced microbial population in highly polluted areas.14
3
Figure 2: (a) salt crust formation on limestone, caused by continuous wetting and drying cycles,13 (b) black crust formation on bricks and mortar
in a highly polluted area,15 and (c) green biofilm on limestone.16
In cities, poor air quality caused by traffic and domestic heating leads to the formation of black crusts on the outer layer of cultural heritage buildings, demonstrated in Figure 2(b). The major component of black crusts is formed by the deposition of SO2 and carbonaceous particles, where the reaction of SO2 with humidity and the Ca in the rock
forms CaSO4·H2O.9,15 The main cause of blackening is elemental and organic based carbon, which originates from soot
particles, as well as CaCO3 in the mortar and limestone rocks.15 There are many factors that affect a stone’s
susceptibility to blackening, including the porosity, how exposed the stone is, and the types of pollutants present at the location.9 The relative amount of carbon and sulphur found in black crusts depends on the substrate on which it is
forming, for example, a carbonate stone or mortar will allow a higher degree of black crust formation. In contrast, a brick which contains siliceous materials, can block the reaction of the CaCO3 binder with sulphurous pollutants.15
Finally, a third danger to cultural heritage buildings is the growth of microbes in the microscopic pores of the stone structure; fungi, bacteria, and algae are all commonly found in historic buildings. All three types of microbe thrive in warm, damp conditions. Deterioration caused by such microbes is commonly found in the form of biofilms, which are well established layers that consist of many different microbes and their extracellular matrices, such as fatty acids, pictured in Figure 2(c).17 Removing them while leaving the surface of the stone intact is incredibly challenging.18
The formation of biofilms is an example of eutrophication. Kobetičová & Černý define the eutrophication of buildings as, ‘(self-)enrichment of building materials by nutrients capable of increasing building biocolonisation and biodeterioration.’18 Eutrophication can occur on buildings that have had nutrients in an accessible form to
microorganisms deposited on them, or on buildings that have degraded to produce these minerals (such as Ca2+, NH 4+
and SiO42--).18 Algae can cause discolouration, and can increase the dampness of a material, and, with fungi and
bacteria, cause major structural damage. Bacteria that cause CaCO3 precipitation through a process called
biomineralisation, cause white stains on the surface of an object, which are challenging to remove using existing methods without damaging the historical surface.19
Fungi are highly durable organisms that can adapt to living on a wide variety of substrates, from stone to wood and paper, by adopting several different metabolomic, structural and morphological systems.16 When present in stones,
fungi can cause both physical and chemical changes, through the expansion of hyphae (the ‘roots’ of a fungus), and mineral disruption caused by the excretion of metabolites and waste from fungal cells.16 Trovão et al. performed a
microbial profiling experiment on the limestone cathedral of Coimbra, in which they identified at least 49 different fungal species, with Aspergillus versicolor being found at all sampling points, and found a taxonomical distinction between biodeterioration types.16 The genera Aspergillus and Penecillum are among the commonly found fungi in
building materials, and the resistance of these fungi to biocidal treatments are standard tests to determine a treatment’s effectiveness.20
Frescoes, and other painted surfaces such as wall paintings, are a particularly demanding conservation challenge, since they are prone to water damage, which results in the surface of the painting flaking off due to salt crystallisation, this bubbling effect is pictured in Figure 20(a,b) in Section 3.2.4.1,124 Frescoes are produced by applying water-based paint
pigments directly onto fresh lime plaster, meaning that the treatments used must reinforce the supporting plaster without altering the appearance of the surface.125
4 Currently, the majority of commercial treatments for the consolidation of stone contain Si, including alkoxy-silane polymers and oligomers, and SiO2 NPs, used for their compatibility with siliceous sandstones. This is demonstrated by
Figure 3, which shows the types of treatments currently commercially available, which mostly have a single, fast acting, consolidating function.21 New treatments being developed are multifunctional, (self-cleaning, antimicrobial TiO2
-containing coatings) and focus on long-term mechanical reinforcement (slow-curing Ca(OH)2 consolidation).
Figure 3: Types of consolidant currently commercially available.21
1.1.2. Wood and Paper
Wood is a versatile raw material that has been used in many different types of historical objects. It has two significant weaknesses, however, that can significantly shorten the lifetime of an object; its susceptibility to biodegradation, and its structural weakness when subjected to variable moisture contents.22 Wood is vulnerable to many types microbe
degradation, the most well-characterised of which are the so-called white, brown and black rot fungi.23 The Aspergillus
species of fungus (Aspergillus niger, Aspergillus versicolor) are a type of black rot fungus, and are the most commonly found fungi on many types of cultural heritage materials, especially wood.18 Figure 4(a) shows bamboo wood which
has been inoculated with Aspergillus niger, which thrives in dark, damp environments and is a frequent problem in cellulose-containing heritage objects, causing a notable decrease in tensile strength as well as visible discolouration.24,25 As well as microbes, wood structures are also vulnerable to larger organisms, such as termites and
woodworm.
Regarding cultural heritage objects made of paper, these are frequently damaged by acidic vapours, which result from the degradation of inks and production chemicals, or by prolonged exposure to light (especially UV), causing fading and discolouration, shown in Figure 4(b). The use of acidic paper that has little resistance to aging was prolific from 1850-1970, and as such there is a huge amount of material that requires treatment. Degradation of both paper and inks from prolonged exposure to light is caused by the photo-catalysed oxidation of organic materials.26 Cellulose
readily depolymerises through hydrolysis when in the presence of acids, which can originate from hydrolysis products of hemicellulose, or acidic additives used in the paper making process, making paper objects particularly susceptible to deterioration.27 The pH of paper can vary across an object, a difference of 1pH unit can be found between inked
5
Figure 4: (a) Brittleness and discolouration of an old book caused by A. niger infection,25 and (b) deterioration by Aspergillus niger on bamboo
wood.29
1.1.3. Textiles
When referring to textiles in cultural heritage, this can apply to clothing and tapestries, as well as the linen used on the back of paintings. The molecular composition of the fibres used in linens can determine its susceptibility to degradation as much as the storage conditions. Maintaining a controlled microclimate for the preservation of textiles is vital, as high humidity encourages the growth of microbes, while higher temperatures can dry out the fibres in the textile, causing brittleness.30 The degradation mechanisms of textiles are generally investigated by artificial aging
experiments where one varies a factor in each case (e.g. water content, humidity). Newer methods of degradation mechanism elucidation involve the application of proteomic and immunology principles to this context.31
Certain natural fibres are more susceptible to degradation than others. Fibres containing cellulose and keratin are more vulnerable than fibres such as silk and lignin, which have more rigid and less hydrolysable molecular structures.32,33 Furthermore, the type of fibre used in a textile determines the type of microbe that can contaminate
it; for example, woollen textiles are made up of keratin, and so are particularly susceptible to keratinolytic organisms.34
The condition of the textile is a strong indicator for the type of microbe present, or the type of microbe likely to infect the object; for example, Egyptian mummies preserved with NaCl were found to have low fungal diversity, with only fungi resistant to high salt concentrations present.35 Thus, the identification of the microbes present on a material
before treatment is vital to determining the biocidal treatment that should be used.
Additionally, a common a line of investigation used in the analysis of ancient textiles is investigating the types of dye that were used in the material. Since many textiles are dyed with organic dyes that degrade quickly, such investigations reveal the original state of the material, and give an indication of the extent of degradation.36
6 1.2. Analysis Methods used in the discussed literature
The field of cultural heritage is highly multidisciplinary, requiring input from many different fields within analytical chemistry, due to the complexity of the materials. Below, the most notable analysis techniques that are used in the papers examined in this thesis are discussed. It is by no means an exhaustive list, but is intended to give some idea of the complexity and level of collaboration required in this field of research.
1.2.1. Methods for the characterisation of cultural heritage objects
Figure 5: SEM images of porous rocks taken by Li et al. (a) shows the filamentous hyphae of fungi penetrating the pores, while (b) and (c) show
spherical calcium carbonate deposits from biomineralizing bacteria.19
Photographic documentation and macroscopic analysis are vital methods of characterising cultural heritage, used to study changes over time, compare type and extent of damage in different areas, and demonstrate from where samples are taken. Figure 2 and Figure 4 show the importance of the visual documentation of damage to cultural heritage objects, demonstrating the nature and extent of the damage of the objects before treatment. X-ray diffraction (XRD) is used to determine different crystal structures of minerals present in rocks, which can give an indication of the origin of the rock.13,37 Light microscopy and scanning electron microscopy (SEM) are used to study the microstructure of
samples, showing, for example, micro-damage or the presence of microbes. Figure 5 demonstrates the use of SEM to image microscopic structures present in porous rocks, including the hyphae of fungi that have infected the stone, and the deposition of calcium carbonate by biomineralizing bateria.19 Energy dispersion using X-rays (EDX/EDS) is used to
determine the elemental composition of a sample in conjunction with SEM results, elements present in a certain spot can be identified, or SEM-EDX mapping can be employed to show the distribution of elements across a sample, a good example of such an analysis is shown in Figure 37 in Section 6.4.35,37 A wide range of techniques is necessary due to
the heterogeneity of many samples, as well as the requirement of different methods to analyse organic and inorganic matter.
Figure 6: Casti et al. used microclimate analysis over a year to demonstrate that the relative humidity varies at different point in the same
geographic location.13
Other tools used in the investigation of cultural heritage can be taken from many different fields. Tests to determine the effectiveness of the treatment must be carefully designed to mimic the conditions of the area in which the object
7 is located.38 Therefore, microclimate analysis is utilised to predict future degradation patterns based on humidity,
temperature, and precipitation.13,30 The valuable information gained from microclimate analysis is demonstrated in
Figure 6, where the relative humidity of four locations at the Early Christian Munazio Ireneo cubicle in Cagliari, Italy, was monitored for a year, and was found to vary significantly within the same geographic location. This shows the importance of understanding microclimate conditions, and how vital it is to consider them when designing a treatment plan.13
Species identification of microbes is performed by swabbing affected areas and growing cultures large enough to identify, which is performed using species specific molecular markers, as well as the morphological characteristics of the culture.39 The use of DNA molecular markers is a common method of identification where, in the case of fungi,
internally transcribed spacer (ITS) regions of the DNA vary significantly between species. In this method, DNA is extracted and amplified using the polymerase chain reaction (PCR), and analysed using gel electrophoresis.40
Additionally, the morphology of certain common microbes, such as A. niger, are well-known and easily identifiable (Figure 4(a)).
1.2.2. Analytical methods to determine the effectiveness of a treatment
The effectiveness of a treatment can be determined using a series of standardised tests. Accelerated aging trials are often used to simulate how a treatment will change over time, these consist of exposing the object to a high degree of light, humidity, and heat. The self-cleaning ability, the ability to degrade solid contaminants on the surface, of photo-catalytically active materials can be measured using an ISO method that considers the water contact angle on the surface.41,42 The colour change following a treatment, can be determined using precise colour change measurements
with colourimetry, following the CIELAB colour change (ΔE*) methodology.43 The colour change is performed using a
colourimeter and calculated using Equation (1, where 𝐿∗refers to black-white colour variation, 𝑎∗to red-green, and 𝑏∗
to yellow-blue.43 A ΔE* value of 5 units or less is commonly accepted in conservation work, as some kind of colour
change is often unavoidable.44 This methodology is also used to determine the self-cleaning ability of a treatment by
quantifying the colour change of the degradation of a coloured dye over time.
∆𝐸∗= (𝐿∗ − 𝐿∗) + (𝑎∗− 𝑎∗) + (𝑏∗− 𝑏∗) (1)
The penetration depth of metal-based NPs can be calculated using laser-induced breakdown spectroscopy (LIBS), where the number of in-depth pulses is related to the penetration depth of the laser pulse, with one pulse reaching to 10µm (See Figure 7(a)).45
Figure 7: (a) LIBS data showing the penetration depth of silver NPs, with intensity corresponding to the intensity of the silver signal, and number
of in-depth pulses corresponding to the penetration depth of the laser pulse, with a depth of about 10µm/pulse achieved.45
The mechanical analysis of rocks is carried out using several standard tests, including water absorption, freeze-thaw resistance, and compression strength.46 For example, measuring surface cohesion is performed by the standardised
‘scotch tape test,’ where simple sticky tape is applied to the surface of the rock, and the amount of surface matter that is removed by tape is weighed.47 Porosity is a key parameter that indicates the strength and stability of stones.
8 Many different techniques are employed to evaluate this. Small angle neutron scattering (SANS) produces a scattering pattern that is fitted to a model that describes the porosity of the stone sample.48 Neutron radiography can be used
to image capillary water absorption of stones, as water is a strong neutron absorber.49 Comparative techniques to
determine the pore size and distribution of porous rocks are mercury intrusion porosimetry (MIR) and low-field NMR. MIR is a destructive technique used to determine the porosity of a sample by gradually injecting mercury into the sample, since mercury is a non-wetting liquid, it will only fill the pores and not be absorbed by the stone.50 In
comparison, low-field NMR is a non-destructive technique, allowing the user to measure the same sample pre- and post-treatment. The pore-size distribution is elucidated by wetting the sample, and analysing the distribution of the relaxation times of hydrogen nuclei.51 The discussed techniques are not an exhaustive list, and are often used in
conjunction to gain a complete picture of the sample.
1.2.3. Characterisation of NP colloids used in cultural heritage treatments
There are several key techniques for the characterisation of colloidal mixtures, since it is important to determine their stability, the size of the NPs, and the dominant polymorph of the material. The colloidal stability can be determined by considering the sedimentation rate, depicted in Figure 8.52 Here, a colloid of 5g/L Ca(OH)
2 NPs was suspended in
varying ratios of water and isopropanol, and allowed to settle for 96 hours, resulting in dramatic differences in the stability of the colloids.52 Another indication of colloidal stability is the zeta potential, which is defined as the electric
potential at the slip plane of a solid/electrolyte interface, assuming the existence of an electrical double layer (EDL). This is measured using electrodes to induce an electrical flow between the colloidal surface and the adjacent fluid, which gives a value for the potential at the position of the double layer where fluid begins to flow.53 From the zeta
potential, one can gain information regarding the net charge density at the solid surface, and therefore its electrostatic interactions with the surrounding environment, such as the thickness of the EDL, and interparticle repulsion due to EDL, which is an indication of colloidal stability.54
Figure 8: A simple method of determining the stability of a colloid, performed using Ca(OH)2 NPs in various isopropanol (IPA)/water mixtures,
96 days after preparation.52
SEM and transmission electron microscopy (TEM) are both used to determine the size distributions of NPs, but are slow and expensive characterisation methods. Figure 9(a) shows a TEM image of a silver NP colloid, and demonstrates the usefulness of the technique when the morphology of the NPs is in question. For the characterisation of metal-based NPs, UV-Vis spectroscopy can be used to directly relate the λmax and width of the absorption peak to the size
distribution of the NPs. A UV-Vis absorption spectrum of a silver colloid is shown in Figure 9Figure 10(b), the unique optical properties of metal-based NP colloids caused by the surface plasmon resonance effect is discussed in Section 2.2.2. Dynamic light scattering (DLS) is regularly used for the characterisation of colloidal mixtures and macromolecules. DLS measures the Brownian motion of particles, by measuring the change in intensity of scattered light over time.55 This change in intensity can be directly related to the diffusion coefficient, which in turn can be used
9
Figure 9: Characterization of silver NPs by (a) TEM,56 and (b) UV-Vis spectroscopy.45
1.3. NPs used in the field of cultural heritage
The following chapters are a discussion of some of the most commonly researched and applied metal-based NP treatments in the field of cultural heritage. Their popularity in the field stems from their established use in other industries, meaning that their production, mechanism of action, and toxicity have been extensively researched in prior literature. The use of silver, calcium hydroxide (Ca(OH)2), magnesium oxides (Mg(OH)2, MgO), titanium dioxide (TiO2)
and zinc oxide (ZnO) will be discussed in detail, as well as a short discussion of novel and less commonly applied NP treatments.
10 2. Silver-based NPs
2.1. Properties
Silver NPs are used in industry for two main properties: their antimicrobial and optical activity. Their use in the medical industry is widely researched for inclusion in antibacterial dressings and drug delivery systems, since their large surface area to weight ratio allows a high density of ligand attachment on the surface.57,58 The shape of silver NPs can be
strongly controlled through synthesis techniques such as chemical vapour deposition onto a structured surface, or alternatively, the reducing agent used in the synthesis can have an effect on the end shape.59 In the field of cultural
heritage, simple spherical colloids of silver NPs are the most commonly used for antibacterial treatments.
The optical properties of silver NPs are also frequently exploited in the pharmaceutical industry, which makes use of the SPR and surface enhancement effect to detect biomolecular interactions, through the use of angle-scanning SPR imaging.60 Surface enhanced Raman spectroscopy (SERS) makes use of this same principle, and allows the detection
of extremely low concentrations of molecules, and is thus commonly exploited in the analysis of the organic components of works of art. Handheld Raman devices have been developed and are currently being investigated for the easy analysis art objects, due to the success of this technique in cultural heritage research.61
2.2. Mechanism of action
2.2.1 Antimicrobial action of silver
The widespread antimicrobial action of bulk silver has been well known for millennia; however, the exact reason for such properties remains elusive. In comparison to traditional antibiotics, silver does not target a specific pathway, and is active against a broad spectrum of microbes. There is extensive research into the elucidation of the many mechanisms of action of atomic silver against microbes. The most commonly suggested include disruption of Fe-S bridges in proteins, exchange with metal catalytic centres of proteins, and interference with DNA.62
Gugala et al. performed a genetic screening study of E. coli bacteria to determine the genes affected by silver and found that a wide variety of cellular processes were involved. Their findings supported many previously proposed mechanisms of action, but also found that silver interrupted processes in the cell wall, ATP production, and the reduction of sulphur.63 Furthermore, Pietrzak et al. studied the toxicity of silver NPs on the fungi Aspergillus niger and
Penicillium chrysogenum, finding significant changes in the metabolism of fungi. Mass spectrometric and qualitative analysis revealed silver-nucleotide adducts and shortening of hyphae respectively. 64,65 Silver NPs have been shown to
significantly inhibit fungal growth, and TEM analysis has shown silver NP aggregation in the cell walls of fungi (Figure 10).56 Figure 10(a) shows the size difference between NPs and fungal cells, demonstrating the ease with which they
are taken up by the cell. Once the NPs are transported into the cell in vesicles, shown in Figure 10(b) they can then be distributed throughout the cell and disrupt vital cell processes including DNA replication and digestion.56
Figure 10: (a) TEM analysis showing silver NP aggregation in the fungal cell wall, and (b) the uptake of silver NPs into the cell.56
From the findings of these studies, one can see the value of the use of silver NPs in the field of cultural heritage. Silver NPs are active against an incredibly wide range of microbes, and target many different cell processes, which means that they can potentially be applied to an object without the need to determine the types of microbe present.
11 However, this does also make predicting a microbe’s resistance to silver NPs more challenging, as the sensitivity of fungi to silver NPs has been shown to change even between strains in the same species of fungus.66
As a general rule, the smaller the NP, the more likely it is to induce apoptosis in cells.67 However, as discussed below,
the toxicity of silver NPs can vary greatly between species, and different species of microbe are susceptible to different sizes of NP, therefore, colloids containing NPs with diameters varying from 10 to 100nm are used in the examples discussed below.68
2.2.2. Surface Plasmon Resonance
The SPR effect is utilised by many different analysis techniques for the detection of low concentration molecules and molecular interactions. Silver NPs are the most commonly used metallic NP in this technique, the SPR effect is caused by the fact that in metal lattices, the electrons are delocalised from the atoms, which has little effect on the properties of bulk materials, but in a NPs this has a big impact on how the electrons and nuclei interact. When excited by electromagnetic radiation, typically UV and visible light are used, these electrons begin to oscillate, and depending on the size of the NPs, at a certain wavelength of incident radiation which depends on the size of the NP, this oscillation will resonate with the light and the oscillation will match the wavelength of the incident light.
However, crucially, the diameter of the NPs is smaller than the wavelength of the incident light, causing the electrons to quantise at the surface of the NP and become plasmons, which then oscillate outside the area of the bulk material of the NP, and begin to interact with materials in the vicinity of the NP. The SPR effect results in the enhancement of electromagnetic fields close to the surface of NP, which is exploited for the analysis of extremely low concentrations of molecules. SERS is commonly used for investigative purposes in the field of cultural heritage, since it is possible to accurately measure incredibly small sample amounts, as the SPR effect amplifies the intensity of the scattered Raman light.69
2.3. Synthesis of silver NPs
Methods of synthesis of silver NPs can vary dramatically, but many methods are based on the aqueous reduction of silver nitrate (AgNO3) using sodium borohydride (NaBH4), in the presence of stabilising agents such as citrate or
tetraethyl orthosilicate (TEOS), as shown in Equation (2.43,70 Surfactants such as oleic acid can be used to prevent the
agglomeration of particles during synthesis.71 Biosynthesis of silver NPs has also been performed by using bacteria to
reduce silver nitrate in a green chemistry approach.25,72
2Ag+ + 2BH4− + 6H2O → 2Ag0 + 2B(OH)3 + 7H2 (2)
2.4. Applications of the antimicrobial activity of silver NPs
Silver NPs are often applied to cultural heritage objects simply by using a brush to apply a colloidal mixture.43 NPs can
also be pre-mixed with mortars and other materials that can be applied as preventative conservation measures.73 A
silver NP misting method has been described by Gutarowska et al., which provides a more uniform application of a colloid. This is performed using a misting chamber (pictured in Figure 11), where the temperature and humidity are tightly controlled, and a silver colloidal solution is sprayed evenly over the desired object. The main benefits of this method are the reduced risk of silver NP inhalation by the user, and uniform coverage of the object.68,74–76
12
Figure 11:Disinfection chamber used by Pietrzak et al. for silver NP misting antibacterial applications.75
A typical methodology to follow when assessing the applicability of silver NPs to an object is to first assess the potency of silver NPs to the microbes found either on the object, or commonly found in museum areas. In this way, treatments are not unnecessarily applied to historical objects.66 There are few examples of pure silver NP treatments applied to
cultural heritage, but there are examples where they are used as a nanocomposite with TiO2 and ZnO NPs to produce
multifunctional coatings, as discussed in Sections 6.6 and 5.5.
2.4.1. Application of silver NPs to paper and textile-based cultural heritage
Paper and textiles are highly susceptible to microbial degradation, and numerous studies on the antimicrobial treatment of silver NPs on textiles have been performed, with visual or mechanical issues upon their applicaition.76
The most common observation across the literature is that the toxicity of the silver NPs is highly dependent on the species of microbe involved. For example, when applied cotton fabric, silver NPs were found to decrease the presence of bacteria by 87-100%, but only decrease the presence of fungi by 32-54%, in both cases the NPs were more effective against vegetative cells than spore cells. This disinfection was performed using the silver NP misting chamber pictured in Figure 11, where a 90ppm colloid containing silver NPs with a diameter of 10-15nm, which was sprayed into the chamber with a relative humidity of 90% over a period of eight hours.76 Such an application results in a very even
coverage of NPs throughout the sample, which can significantly enhance the antimicrobial properties of the treatment. In order to avoid unnecessary applications of silver NPs to historical objects, filter paper is often used as a test surface when studying fungi found on cellulose based materials. It has been shown that the simple application of silver NPs to the filter paper will increase its mechanical strength, indicating that there is also a significant interaction between the NPs and the paper, as well as with microbes, although the nature of this interaction is not clear.25 Furthermore, silver
NPs have been found to be highly potent to fungi that grow on paper, and were able to inhibit the growth of the Bacillus fungus on paper by up to 100%.77 Increasing the concentration of NPs increases the potency of the treatment,
however, one must find the balance between effectiveness, cost, and appearance.77
The applications discussed above were tested on reference samples of textile and paper, there are few examples of the application of silver NPs to cultural heritage for the purposes of antimicrobial treatments. Many studies on the antimicrobial properties of silver NPs are performed based on the ability of a NP to inhibit the growth of a species of microbe, a preventative treatment, however when applying such treatments to cultural heritage, one must also consider its ability to treat materials that are already infected, corrective treatments.78 When applied to pre-Columbian
archaeological textiles, the observed potency of silver NPs to established microbe cultures was significantly lower than that observed when investigating the preventative ability of silver NPs. The textiles investigated included wool and cotton, and silver NPs decreased the presence of microbes by between 30% and 99%, but this number strongly depended on the presence of different microbe cultures, with endospore-forming bacteria being minimally affected.75
Such findings indicate the importance of preliminary tests into the biocidal activity of silver NPs against a particular mould, to avoid the unnecessary application of ineffective treatments.
13 2.4.2. Application of silver NPs to stone and cultural heritage monuments
Figure 12: The yellowing of white stone is caused by silver NPs when used in combination with a variety of stabilisers and other NPs.43
The yellow staining of stone surfaces, pictured in Figure 12, caused by both pure silver NPs and silver NPs as part of a nanocomposite is a well-documented issue, likely due to their optical activity, therefore their application to bright white rocks is discouraged.43,79 For instance, in accelerated aging trials of white and autoclaved aerated concrete, in
comparison to a range of commercial treatments, the use of silver NPs showed the biggest visual difference, with significant surface darkening observed.80 A reduction in the darkening effect has been observed upon the citrate
stabilisation of silver and silver/TiO2 nanocomposites due to reduced NP aggregation on the stone surface, but the
colour-change is still more apparent for silver NP treatments in comparison to other NPs used.73
Despite the unfortunate staining action of such colloids, they have proven to be the most effective inhibitor of algal growth on limestone substrates, when compared to a variety of TiO2 and ZnO colloids and nanocomposites, both on
damp substrates and when fully immersed in water, with dispersions of 20ppm proving to be effective, such nanocomposites are discussed in Section 5.5.2.43,67 In many cases, silver NPs are functionalised during synthesis with
TEOS, which then will react with the siliceous material in the stone to ensure that the NPs adhere to the stone surface, which makes the treatment more resistant to leaching than using pristine silver NPs.43,70 However, the common issues
with silver NPs as a corrective treatment continue to be reported when applied to stone, for example, silver NPs were applied to inoculated archaeological stone, and the treatment had a high toxicity against the bacteria P. carotovorum, but no effect on the fungus A. alternata.78
2.5. Applications using SERS to analyse cultural heritage objects
SERS is incredibly useful for the analysis of organic pigments and binders in artworks, there are many examples of SERS being used for this purpose, a few of which will be discussed here. There are proposals to create a database of SERS spectra of common dyes found in cultural heritage, since the identification of many organic dye in this field is based on comparisons to reference spectra, however, there can be significant differences between recorded spectra depending on the material and colloid used, which should all be included in such a database. 81
Research performed at the Art Institute of Chicago investigated the pigments present in the sky of Wilson Homer’s watercolour painting, ‘For to be a farmer’s boy,’ pictured in Figure 13(a). It has long been suspected that the pigments present in the sky have faded, and that it was originally intended to depict a sunset scene. Pigment grains were removed from the sky area, and SERS was performed using a Raman microscope, simply by dropping a paste containing a silver colloid onto the pigment particle, and recording the spectra. The obtained spectra were then compared to SERS spectra of known organic dyes used in the same time period, and the comparison shown in Figure 13(c) indicates that the organic dyes cochineal, burnt carmine, and Indian purple were used in the painting.
14
Figure 13:Research into (a) Wilson Homer’s ‘For to be a farmers boy’ reveals that (b) the sky was originally a deep orange sunset, which was
determined through (c) the use of SERS and comparison of spectra to known 19th century organic paint pigments.82
The analysis of cochineal dyes is very common with SERS, since it is very difficult to measure this molecule using normal Raman spectroscopy due to fluorescence from paper and binders. SERS is a technique that can quench fluorescence, and thus allows users to fully characterise object that contain organic pigments that would otherwise go undetected. Cochineal was detected using SERS as a complimentary technique to analyse traditional Chinese Lajian paper from a single fibre. Like with the work discussed above, the cochineal dye was identified through a comparison of standard spectra.83 A similar methodology is pictured in Figure 14, where the SERS spectra of an
archaeological textile and a reference of the dye alizarin are compared.36 This emphasises the need for a complete
database of SERS spectra for the analysis of organic dyes in cultural heritage, especially since organic dyes are susceptible to a wide range of degradation pathways, and therefore references in many different degradation states must be considered.
15 The application of silver NPs to cultural heritage for the purpose of antimicrobial treatments is rarely documented, but their use in SERS investigations is more commonly reported. There are no recent examples of pure silver NPs being applied to stone buildings, most likely due to the discussed yellowing effect, and their use in multifunctional coatings such as TiO2 and ZnO-containing treatments. The biggest benefit of the use of silver NPs is their widespread
antimicrobial action, therefore they can be used as a preventative treatment or for heavily infected objects. On the other hand, the widespread microbial action of silver means that the mechanism of action is still not understood completely, and there is little understanding of how the NPs interact with the substrates to which they are applied. In addition to this, an issue that will be repeated frequently in this thesis is that since the use of silver, and other, NPs is a novel concept for cultural heritage conservation, and there are no reported studies on how silver NPs will react with the materials they are used to treat in the long term. This means that any application of NPs to heritage materials must be monitored closely over time to track any indication of accelerated degradation due to the treatment.
16 3. Calcium Hydroxide
3.1. Properties
The use of Ca(OH)2 NPs, also known as nanolimes, is very common in the building industry for stone and concrete
consolidation. Ca(OH)2 NPs are most commonly found as hexagonal crystals, a polymorph called Portlandite, shown in
Figure 15.84,85 Ca(OH)2 NPs are basic, and thus can be used as a deacidification treatment. However, they are more
commonly applied to calcium carbonate-based materials, where they then react with CO2 to form CaCO3, in a process
known as carbonation. There are two commercially available products for cultural heritage conservation containing Ca(OH)2 NPs; CaLoSil and NanoRestore. Both products are available in a variety of solvents and concentrations, with
CaLoSil NPs having a diameter of 150-250nm, and NanoRestore 100-300nm.6,84,86 Such products are used on
calcium-containing rocks, such as limestone, sandstone and marble, and have been shown to perform significantly better as consolidants than traditional acrylic resins.87
Figure 15: Field Emission-SEM images of Ca(OH)2 NPs, showing the hexagonal Portlandite crystal structure.85
In the context of stone consolidation with nanolimes, the properties of calcium carbonate are important to consider. Calcium carbonate exists in three crystal structures, the most stable and commonly found in nature being calcite, with the metastable minerals aragonite and vaterite also present.88 Calcite is formed in geological processes, and is the
main structure found in building materials, while aragonite and vaterite can be formed by biological processes, such as from CaCO3-producing bacteria.89,90 The three polymorphs are pictured in Figure 16, finely ground crystals of
aragonite are pictured in Figure 16(a), which naturally grow a needle-like structure, which facilitates the penetration of carbonate stone structures to effectively cement the material. Figure 16(b) shows the cubic crystals of calcite, the most stable of the three polymorphs, and Figure 16(c) shows the subspherical, disorganised structure of vaterite.48
17 3.2. Synthesis of Ca(OH)2 NPs
The synthesis of Ca(OH)2 NPs is very simple, due to the poor solubility of Ca(OH) in water; therefore, upon the reaction
of NaOH and CaCl2, the Ca(OH)2 will simply precipitate out of solution, as shown in Equation (3.91 The NaCl must be
removed from the solution via a washing step, as the presence of salt could further damage the surface of a historical object.92 The precipitate can be washed using an ultrasonic bath to remove the NaCl, dried, and resuspended in a
solvent of choice at the desired concentration.52
2NaOH(aq) + CaCl2(aq) → Ca(OH)2(s) + 2NaCl(aq) (3)
The properties of nanolimes can be tailored for the required application; for example, precipitation of Ca(OH)2 NPs in
the presence of cellulose nanofibers can enhance their compatibility with paper-based materials.93 Furthermore, Zhu
et al. showed that the dimensions of Ca(OH)2 NPs can be controlled, by performing the reaction in the presence of
dimethyl formamide (DMF) and octyl phenyl polyoxyethylene ether (OPPE) surfactants. Increased [OPPE] causing wider and flatter crystal growth, and increased [DMF] encouraging thinner, needle-like crystals, as visualised in Figure 17.94 Such a reaction can be used to tailor a treatment to a specific application, depending on the porous structure
and damage of the substrate.
Figure 17: Schematic demonstrating the effects of [OPPE] (a-d) and [DMF] (e-h) on the nanolime dimensions.94
Finally, Taglieri et al. have patented the production of Ca(OH)2 NPs through the use of an ion exchange resin. CaCl2 is
mixed with a strongly basic ion-exchange resin, and the hydroxide groups of the resin exchange with the chloride ions, thus negating the washing stage of the synthesis, and increasing the purity and yield of the synthesised particles.95–98
The benefit of these NPs in comparison to those available commercially is that the synthesis method described is milder than industrial production methods, the NPs produced are consistently of high purity and a similar shape (<10nm), independent of the reaction time.96 These NPs have been applied to various historic building and frescoes,
the results of which are discussed below. 3.3. Mechanism of Action
3.3.1. Deacidifying action of Ca(OH)2 NPs
Calcium is an alkaline earth metal, meaning that it readily reacts with water to form Ca(OH)2, an alkaline compound
with a pH of 12.4 upon dissolution in water. This in turn will readily neutralise in the presence of an acid. Example reactions are shown in Equations (4 and (5. The acidification of paper is a common issue caused by the products of cellulose degradation. Nanolimes are used as a conservation treatment due to their alkalinity as they can neutralise acids with the formation of water as a by-product. Furthermore, the insolubility of nanolimes in water creates an ‘alkaline reserve,’ making the nanolimes act as a long-term treatment.
Ca(OH)2 + H2CO3 → CaCO3 + 2H2O (4)
Ca(OH)2 + 2HCl → CaCl2 + 2H2O (5)
3.3.2. Calcification of Ca(OH)2 NPs in stone and mortars
Mortars are made up of lime, water and aggregates, generally fine sand. When they set, two reactions are occurring, the calcification of Ca(OH)2 into CaCO3, and the slower pozzolanic reaction, where the alumina and silicate materials
in the sand react with the lime to form calcium silicates.99 These two reactions form materials that have high
18 Nanolimes are commonly used to consolidate calcareous stones such as limestone and marble, but in principle can be applied to any porous material for consolidation purposes.100 Since the nanolimes calcify into CaCO3, they are
incredibly effective at cementing the substrate together and filling any cracks that are causing the mechanical strength of the stone to decrease, and since, chemically, the two materials are identical, there is a very strong interaction between them. The basic reaction of the calcification of nanolimes is as follows:
Ca(OH)2 + H2O + CO2 → CaCO3 + 2H2O (6)
The rate and efficiency of this reaction is determined by a number of factors, firstly, the nanolimes must dissolve in water that has been absorbed by the porous material, and secondly, CO2 gas must also dissolve in this water in order
to form carbonate (CO32-) ions.100 This newly formed CaCO3 ‘cement’ is then able to bind together loose grains in the
stone and fill cracks. An examination of the nucleation and rate of Ca(OH)2 carbonation found that initially, the rate of
nucleation was far greater than that of crystal growth, meaning that the CaCO3 crystals formed are very small and can
fill microscopic cracks and pores.101 This indicates that for many nanolime treatments, the initial calcification is a quick
reaction, and that such treatments can have immediate results.
Once the carbonate cement is formed, however, the curing process continues. When nanolimes calcify, they are first in an amorphous calcium carbonate phase, over time, this reorders into vaterite, which then dissolves and precipitates into calcite over a longer period of time. This has been determined using crystallinity studies, and brings to light the dynamic nature of the system, and the need for a conservation material that will change with the substrate, in order to avoid accelerated degradation.100,102–104
In addition to this, additivation of nanolimes with carbonate polymorphs, such as calcite and vaterite, has been shown to increase the rate of calcification, by providing nucleation points for the reaction.48,105 It is challenging to determine
the success of a consolidation treatment of Ca(OH)2 NPs, since the consolidation products formed are chemically
indistinguishable from the carbonate stone on which they are applied. Spectroscopy is often used to monitor the rate of calcification as an indication of a treatment’s success.106
Spectroscopic studies into the mechanism of carbonation of nanolimes has revealed the formation of calcium alkoxides as an intermediate product when the NPs are applied in an alcoholic solvent, and that the presence of these alkoxides can increase the rate of carbonation significantly.104 A CO2 insertion step then leads to the formation of
CaCO3, as shown in Equation (7, and thus this insertion decreases the activation every required to for CaCO3.103 Based
on this evidence, calcium ethoxides have been developed as a potential consolidation treatment.
(7)
3.4. Consolidation of stone with Ca(OH)2 NPs
When testing new consolidation treatments, it is very common to use stones from a quarry that is known to have been used in the construction of historic buildings, allowing researchers to perform extensive tests on stones identical to those used in historic buildings without damaging the surface of a monument.44,50,107
The application of Ca(OH)2 dispersions can be as simple as using a paint brush or a spray bottle.108 Other methods of
application include the use of rollers and airless spray, where the penetration depth of the NPs can be controlled by the number of applications.106 The treatment can be tuned to the application by changing the solvent, NPs dispersed
in isopropanol or water are effective at filling smaller pores (0.05-0.7µm), while mixtures of water/ethanol and water/isopropanol cause larger pores to be filled (17-100µm).109 Thus, the pore size must be taken into account when
applying a nanolime treatment, i.e. by using a larger NP for a stone with larger pores for maximal effectiveness, and to avoid reducing the porosity of the stone too much.110 It is still necessary to maintain some porosity in a stone sample,
as the presence of trapped water in a stone after a treatment could have significantly negative effects in the case of freeze-thaw cycles.12
19 3.4.1. New Ca(OH)2 NP treatments not yet applied to cultural heritage
The long-term effectiveness of nanolimes as a consolidation treatment depends on their penetration depth into the stone. The depth of penetration is dependent on NP diameter, the solvent used in the application, and the porosity of the stone. The penetration depth of Ca(OH)2 NPs into porous limestone can be shown using the indicator
phenolphthalein, which turns pink in the presence of alkaline substances. This method of determination of the penetration depth is visualised in Figure 18(a-c), which makes clear the significantly different results obtained by treating different types of stone with nanolimes.111,112 Figure 18(a) shows nanolimes that have migrated evenly
throughout highly porous Maastricht limestone, while in Figure 18(b,c) the penetration of the nanolimes is severely hampered by the much harder adobe and Tabaire rocks.111,112 The fluorescence of zinc quantum dots has also been
used to demonstrate the varying penetration depths of different Ca(OH)2 NP treatments, visualised in Figure 18(d,e).113
Here, Figure 18(d) shows the penetration of Ca(OH)2/ZnO stabilised by (3-aminopropyl)trimethoxysilane (APTMS) and
Figure 18(e) that of Ca(OH)2/ZnO stabilised by TEOS.113 Here, the two stabilising agents have had a big effect on the
penetration depth of the nanolime colloid, due to the difference in colloidal stability caused by the stabilising agents. The colloid stabilised by TEOS was found to be much less stable than that stabilised by APTMS, determined by sedimentation rate studies in different solvents. The colloidal stability has an effect on the penetration depth, as the more stable the colloid, the more the NPs are able to travel through the pores of the stone.113,114
Figure 18: The use of phenolphthalein indicator to visualise the penetration depth of Ca(OH)2 NPs in (a) highly porous Maastricht limestone,111
(b) adobe and (c) Tabaire.112 The use of zinc quantum dots can also demonstrate the penetration depth of a treatment limestone, using (d)
Ca(OH)2/ZnO stabilised by (3-aminopropyl)trimethoxysilane and (e) Ca(OH)2/ZnO stabilised by TEOS.113
Ca(OH)2 NPs can increase the mechanical strength of limestones and other calcareous stones, which is attributed to
the high compatibility of the treatment with the substrate surface.92 The success of a treatment is determined by
testing the stone’s durability through freeze-thaw tests, micro-drilling resistance, and salt crystallisation cycles, among many others.44 However, a balance must be found between improving the mechanical strength of an object and
affecting its visual appearance, as the mechanical properties of a stone will improved with the continued addition of nanolimes, even beyond recommended dosage rates.108 Over-application can result in an observed whitening of the
surface of the stone, as well as a reduction in the treatment efficiency, as increased concentrations will cause the NPs to agglomerate and block pore networks.87,107
There are many different aspects to consider when designing a new consolidation treatment; it is important to consider the desired outcome of the treatment, as well as the environment in which the object is kept. For instance, the use of fast drying solvents can result in the visual whitening of coloured surfaces, as there is little time for NP dispersion into the stone before the solvent evaporates.112,115 In addition to Ca(OH)
2 NPs, TEOS is also commonly
20 mechanical properties, due to the ability of TEOS to penetrate pores less than 50nm and interact with silica compounds in the substrate, and Ca(OH)2 to bind together calcium-based materials on a microscopic level.116,117 Thus, the nanolime
treatment applied to a heritage monument constructed from calcareous stone should be tailored for the specific stone, taking into account pore size, humidity and chemistry.
Nanolimes can be used in conjunction with other nanomaterials for additional protective properties as well as to increase the compatibility of other treatments with a substrate. For the consolidation of wall paintings on calcium-based substrates, Ca(OH)2 NPs are often used due to their compatibility with the surface of the painting. Erosion
caused by dampness means that the painting surface no longer adheres to the stone wall.118 Zhu et al. have found that
the combination of Ca(OH)2 NPs with 2D nanostructures such as graphene quantum dots (GQDs) and boron nitrides,
improve the fire retardancy of wall paintings by establishing a gradient penetration depth structure.119 Furthermore,
synthesising Ca(OH)2 NPs in the presence of GQDs results in smaller particle sizes, accelerates the carbonation process
through the capture of CO2 with graphene, and demonstrates a novel UV-damage protection ability.120 Such a complex
system is an example of how the field of cultural heritage conservation can benefit from advances in other fields of nanotechnology.
The European collaboration NANOMATCH aimed to contribute to the increasing issue of built cultural heritage degradation by developing a new class of consolidants for stone, wood, and glass substrates. Within this project, calcium ethoxide (Ca(OEt)2) has been developed as an alternative nanoconsolidant to Ca(OH)2 for carbonaceous
stones. A comparison between the two is shown in Figure 19, where the Ca(EtO)2 seems to preserve the surface
porosity of the stone far better than the traditional nanolime treatment, possibly due to an increased colloidal stability in comparison to traditional nanolimes. As discussed, the carbonation reaction of calcium alkoxides is faster than that of Ca(OH)2, and the formation of calcite is observed as quickly as a month after application.103,104,121 The reason for
such different carbonation behaviours is likely due to the fact that here, two reaction pathways are present, either through CO2 insertion (Equation (7), or through first the formation of Ca(OH)2 and the subsequent carbonation.103
Additionally, the solvent with which it is applied affects the rate of carbonation, with OH-containing solvents increasing the rate in comparison to non-OH-containing solvents, another example of the possibilities available to fine-tune nanolime treatments to the application and its enivronment.122
Figure 19: A comparison of the surface morphology of treated stones. Ca(OEt)2 seems to preserve the surface porosity of the treated stone far
21 3.4.2. Case studies: Applications of Nanolimes to Cultural Heritage buildings
There are several examples of the experimental application of Ca(OH)2 NPs to cultural heritage monuments. The
availability of the commercial consolidation products CaLoSil and NanoRestore indicate the widespread use of nanolimes for restoration, examples of which are discussed below.
The stones of the San Massimo d’Aveja Church in Italy are ideal candidates for treatment with nanolimes, since both the mortar and stones used are lime and calcium carbonate based, removing any compatibility issues. After treatment with NPs produced using the ion-exchange resin method of Taglieri et al., it was found that the porosity had been reduced by only 15%, but the amount of water absorbed by some stones was reduced by up to 60%.107 The same
nanolimes were used for the restoration work of the L’Aquila church in Italy. Taglieri et al. used Ca(OH)2 NPs specifically
designed for the unpainted stones used in the building, no colorimetric discolouration was observed, and significant improvements in mechanical properties were reported.123 This indicates that the nanolimes produce using this method
effectively consolidate in areas of damage but don’t significantly alter the macroscopic structure of the stone.
Figure 20: (a)A 16th century fresco in the Cathedral of Florence, (b) a close -up image of the damage to the painting surface, and (c) a close-up
image of the painting after treatment with colloidal Ca(OH)2 NPs.124
Ca(OH)2 NPs have been successfully used in many examples to reinforce the structure of frescos, since they can
effectively penetrate below the painted layer of the plaster. The group of Piero Baglioni have demonstrated that Ca(OH)2 are effective consolidants for frescoes, notably applying this treatment to the Mayan wall paintings at the
World Heritage Site in Calakmul, Mexico. An early example of the restoration of frescoes with nanolimes reported by this group is in the 16th century frescoes in the Cathedral of Florence. Here, the treatment was applied to an area of
the painting that had been damaged by damp Figure 20(a,b), to reattach detached paint layers to the wall surface. The final effects of the treatment, Figure 20(c) show a dramatic difference in the appearance of the painting, with the paint layer suitably stabilised on the wall surface.124
22
Figure 21: The treatment procedure used for the restoration of the Mayan wall paintings in Cakalmul, Mexico.126
There are many example of Mayan wall paintings throughout South America, the paintings treated by the Baglioni group were discovered in Calakmul, Mexico, in the 1940s, and in 1996, the first conservation treatment was applied, using an organic polymer as a protective layer on the surface of the paintings.127 However, this polymer material was
not designed for the harsh climate, and the coatings accelerated the degradation of the paintings.126 The restoration
of the Mayan wall paintings was reported by the Baglioni group from 2006 onwards, and a specific restoration procedure was developed. First, the polymer coating was carefully removed, using a specially developed nanostructured detergent system, the resulting paint layer was very unstable, and was flaking from the surface of the stone, or was becoming a powder, and was easily removable with a cotton swap (Figure 21(c)).127 Then, an aqueous
colloid of Ca(OH)2 NPs was applied using a paintbrush (Figure 21(a), and a wood-fibre poultice was applied for 8 hours
in order to keep the surface wet, and allow the NPs to disperse throughout the plaster (Figure 21(b)).126 This procedure
was repeated a second time, and then it was shown that the paint layer could no longer be removed with a cotton swab (Figure 21(d)), and the image of the painting was much clearer (Figure 21(e)). Images using grazing visible light are shown in Figure 22 to demonstrate the extent of the damage before and after restoration of another painting on the site. Figure 22(b,d) clearly show the paint layer flaking from the surface of the plaster, and Figure 22(c,e) show the effective consolidation that the treatment provided.114,128
