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Introduction to problem of art research
In today's society cultural heritage, buildings, art, and also nature, has a big impact. For example, artworks are seen and loved by many people in museums, churches, and in galleries. Art can be damaged by e.g. usage, storage, or decay processes of the materials used induced by e.g. light or humidity, therefore art is studied for conservation and preservation. In this field three groups of scientist are working together; art historians, conservators, and natural scientists.
For conservation and restoration analyses on artworks are necessary. Since non destructive in-situ techniques, directly on the artobject in the museum, are not widely spread samples need to be taken from the art works. These very small samples are taken destructively from art objects to disturb the integrity as less as possible. Usually, samples are taken from a part of the artwork that is not directly visible, e.g. for paint samples they are taken from the side of the canvas. Some of the analyses on these small samples are non destructive, as raman, though the most used techniques are destructive techniques and duplicate analysis is not always possible due to sample size. For preserving the object in situ techniques are best to use, when not available next steps are non destructive analysis and last step is destructive analysis. [1-2]
In the period 1856-1900 synthetic organic dyes became widely available and were mainly used for textile dying and paint. For preservation and conservation reasons it is interesting to have a method that easily classifies the dye class or more preferably determines the dye itself. Hitherto these dyes are only investigated with HPLC, 3D fluorescence and MALDI-TOF. [3-4]
Synthetic organic dyes
In the eighteenth century two semi-synthetic organic dyes were discovered by treatment of natural indigotin with nitric acid and sulphuric acid to form indigo carmine (1740) and picric acid (1777), respectively.
William Henry Perkin invented the first synthetic organic dye by accident whilst working on a route to synthetic quinine as an anti-malaria drug. In August 1856 William Perkin applied for a patent for this synthetic organic dye known as mauve and since 1857 it is commercially produced. France and Britain were the first to produce more synthetic organic dyes, Germany and Swiss followed in the 1860's. In these years more different dyes became commercially available. 
In total about 400 synthetic organic dyes were discovered in the twentieth century, but many of them were never brought onto the market since they were not cost-effective nor had satisfactory properties. Complicated mixtures of dyes are not expected to be present in artworks from this period, because there were many hues of all colours available. 
Next to dyes there are also pigments, they are different on application to a fibre.
Dyes are soluble and are applied to a fibre in an aqueous solution. These dyes are designed to be attracted strongly to the polymer molecules which make up textile fibres. Contrary pigments are generally insoluble, and have no affinity for the fibre. Pigments are attracted to one another in their solid crystal lattice structure in order to resist dissolving in solvents. By precipitating dyes with a metal salt pigments are formed called lake pigments. 
The colour and hue of colorants, dyes and pigments together, are determined by the chromophore and the auxochrome respectively.
The chromophore of dyes is the region in the molecule with conjugated pi systems, for pigments it is a metal complex. This conjugated pi system absorbs visible light to excite electrons to a higher molecular orbital .
An auxochrome consists of functional groups with non-bonded electrons. The auxochrome is connected to the chromophore by benzene or naphthalene rings, the chromophore and these rings form the chromogen. The absorption of light will be intensified by this direct conjugation of the auxochrome with the chromophore. It may increase the wavelength at which the light is absorbed as well, this is called a bathochromic shift, the reverse effect is called a hypsochromic shift.
All colorants are divided in subclasses based on their chromophore and auxochrome. In these classes a further distinction is made between dyes and pigments and also on binding mechanism to the fibre. Dyes can be applied to a fibre in an acidic medium and therefore called acid dyes, which is the most important group. Likewise dyes can be applied in a basic medium named basic dyes, mordant dyes are bonded to the fibre via a transition metal and direct dyes bind with a hydrophobic interaction in a neutral aqueous environment. This also applies for pigments. [3, 6]
The German Dr. Lehne wrote the book, Tabellarische Ubersicht über die künstlichen organischen Farbstoffe und ihre Anwendung in Färberei und Zeugdruck. This book is about synthetic organic dyes from the period 1856-1900. His first book was published in 1893 with 329 dyes and in 1899 he published a second book with 186 new dyes. For all colorants he has written about, a swatch, recipes, and properties are added. One of the properties he wrote about was light fastness. Dr. Lehne determined light fastness by putting a swatch of dyed textile in front of a window and described the colour change over a defined period.
The book Colour Index of Society of Dyers and Colourists and the American Association of Textile Chemists and Colourists lists all synthetic organic colorants to around 1970. This book consists of five volumes and is printed in 1971. The Colour Index name, commercial name(s), discoverers, most of the structural formulas, and all known manufactures are depicted. 
The Schweppe Collection
There are three institutes in the world that possess the Schweppe collection, viz. The Netherlands Institute for Cultural Heritage (ICN), Amsterdam, the Royal Institute for Cultural Heritage in Brussels, Belgium, and the Tate Gallery, London, UK.
The Schweppe collection is a selection of 65 synthetic organic dyes given by Dr. Helmut Schweppe from BASF, Ludwigshafen, Germany. This collection consists of the most important dyes from the period 1856-1900, both pure dyes and dyed wool, and covers most dye classes. 
Synthetic organic dyes are an unexplored area in cultural heritage research. The subject of this research is part of a project about history, applications, properties, and decay of synthetic organic dyes. All research in this project is performed on the selected 65 synthetic organic dyes by Dr. Schweppe. The following will be about the synthetic organic dyes from the Schweppe collection, unless otherwise stated.
The ICN used High Performance Liquid Chromatography (HPLC) and 3D fluorescence to investigate these dyes  and The Tate Gallery in cooperation with Simmons College and Museum of Fine Arts, Boston, USA analysed the same dyes with Matrix-assisted laser desorption/ionisation-time of flight (MALDI-TOF). 
3D Fluorescence spectroscopy is a non destructive technique that measures the emission and excitation spectra simultaneously and combines them into a three dimensional spectrum. The 3D fluorescence spectrum is, as emission and excitation spectra are, not very specific these spectra usually have broad bands. When a minor difference in multiple molecules is present, as is common for dyes, in 3D fluorescence spectrum might no difference visible. Also spectra for one dye can be changed by chemical treatment, difference in pH, which is common when dyes are attached to fibres, or the substance itself, when it is changed by for example degradation, common for old samples.
Though 3D fluorescence is useful as a first non destructive scanning technique to minimize the number of scans and hence the amount of sample for HPLC. If the dye is present in low concentrations on for example a fibre or in a combination of dyes it can be impossible to detect the dye.
HPLC is a destructive technique that uses an apolar column to separate different components, hence it is particularly useful for mixtures of dyes. With HPLC all dyes can be determined, though it requires different systems for basic and acidic compounds consequently some dyes cannot be distinguished in one single run. Since it is a macro destructive technique big samples are necessary for duplicate analysis. Dyes from wool fibres are extracted with hydrochloric acid and analysed with HPLC. 
MALDI-TOF is useful for a rapid and reliable determination of dyes. A dye solution is mixed with an ultraviolet-absorbing compound. The liquid is evaporated and only a mixture of fine crystals of matrix plus analyte is left. A very short UV laser pulse absorbed by the matrix will evaporate matrix-ions into the gas phase. The matrix that carries the analyte will distribute ions from the matrix to the analyte and the analyte expands. After this expansion a voltage pulse expels ions into the spectrometer. The time of flight instrument separates ions with the same kinetic energy but different m/z, heavier ions need more time to travel.  When using negative and positive ion spectra structural information is provided and the exact dye can be determined.
The ICN also analyses dyes with Fourier-Transform Infrared Spectroscopy (FTIR). Which is a convenient technique for pure dyes, but for dyed textile, textile overrules the dye signals in the spectrum.
A combination of mass spectrometry methods, FTIR, Thin film Liquid Chromatography (TLC), UV-Vis, and HPLC is used to determine used dyes. Problem with these techniques is that they are destructive and TLC and UVVIS are not specific enough. A solution to this could be Raman spectroscopy because it is non-destructive. The ultimate goal would be an in-situ Raman technique.
Theory of Raman spectroscopy
Sir Chandrasekhara Venkata Raman, 1888-1970, discovered the Raman effect in 1928. In 1930 he won the Nobel Prize in Physics "for his work on the scattering of light and for the discovery of the effect named after him". 
Raman scattering is inelastic scattering of the incoming laser light and is present in the Stokes region of the spectrum. This effect is very weak, approximately one of 108 photons is a Raman photon.
To obtain Raman scattering functional groups of the molecule need to be polarised. Polarisation occurs when charge centres of functional groups are distorted by the electromagnetic field of light and cause an electric dipole moment. The strength of polarisation is determined by field strength of the laser light and also on the rigidity of the molecule. The ease of polarisation is called polarisability and this can change due to internal molecular vibrations in magnitude and direction, caused by other excitations of the molecule. With molecular vibrations the polarizability changes periodically
Selection rule: raman active change in polarization, due to vibration, change in polarization can be reflected by a change in either magnitude or direction of polarizability. IR rule molecular vibration should induce change in dipole moment.
In contrast with IR Raman is useful for measuring vibrations of non-polar bonds because they are usually polarisable. 
For this research functional group analysis is very important because many dyes have minor differences in their molecules and also same functional groups on different positions in the molecule are possible to detect.
Theory of Surface Enhanced Raman Scattering applied to art research
In 1974 Fleschmann et al observed something like sers and firt article.. 
OVERVIEW OF 90'S WHERE many researchers were working on sers to explore the nature of this phenomenon.
 M. Moskovits, Rev. Mod. Phys. 57 (1985) 783.
 A. Otto, in: M. Cordona, G. G.untherodt (Eds.), Light Scattering in Solids IV, Topics in Applied Physics, Vol. 54,
Springer, Berlin, 1984. 
Beetje over hoe sers werkt verschillende mechanismen, met plasmon en waarom het van nut kan zijn. Principles of SERS lezen.
Best sers probes are dyes that fluorescence the most. (bron)
In artikel waarom R6G 
Theorie uit artikel:
Two main contributions:
- local enhancement of incident optical field by protrusions or in cavities of rough surfaces or clusters of colloidal partical à EM enhancement = generally accepted.
- callled chemical enhancement arises from interaction of molecules with metal and is responsible for a dramatic sers enhancement can be observed
Fluorescence is quenched due to by through space long range energy transfer of perrin-forster type if dye molecules are adsorbed at surface.
To this phenomenon there are a few contributions, the first is generally accepted. Electromagnetic enhancement due to local enhancement of the incident optical field, laser light, by protrusions or in cavities of rough surfaces or clusters of colloidal particles.
The second contribution is called chemical enhancement arises from interaction of molecules with metal and is responsible for a dramatic SERS enhancement can be observed
Sm sers: 3 contribution: Resonance between light and electronic transition
(dis)advantages of the use of Raman in Art research
With the introduction of better equipment Raman technique became more useful in many fields. A microscope connected to the Raman system, already mentioned by Dhamelinourt et al in 1979 , could measure samples within the spot size range of microns. Also better detection systems such as CCD's, improved lasers with different initial wavelengths, better tuneable and lower adjustable laser power, and better spatial resolution results in higher quality spectra. Due to the lower laser power it is also possible to measure unique samples non-destructively. Since fibre optics became more suitable it can be used for in situ Raman spectroscopy. 
In situ measurements are advantageous because art objects are certainly not damaged by transport. nevertheless it is difficult to have a portable instrument as good as one from the lab. [16-17]
The most important advantage of Raman spectroscopy is no sample preparation especially useful when dealing with microscopic unique samples and duplicate analysis in not possible. It is also profitable that analyses are not performed in vacuum, glass sample holders can be used and water is practically invisible because O-H bonds are difficult to polarize .
Glass gives very little Raman scattering because it is hardly polarisable. When dealing with such small amounts of sample it is preferred to prevent any interaction of glass, which is observed in the fingerprint region of dyes, by placing samples on aluminium. Aluminium is even more difficult to polarize. 
Synthetic organic dyes have conjugated benzene systems which is rather good for Raman, because they are highly polarisable due to unlocated electrons, on the other hand that can also cause fluorescence, which is a major disadvantage. Fluorescence does overrule the Raman signals because the effect is very weak.
Fluorescence is appears as a large featureless background that masks Raman bands from the sample being analysed. Fluorescence is a radiative decay process occurring in excited state with same quantum number that occurs after high intensity NIR, visible, or UV light has excited electronic molecular states. It can, in some instances, be reduced, or removed altogether, by changing the initial laser wavelength, heat regulation, photobleaching or application of metal colloids for SERS. [11, 20] Fluorescence is most common with organic substances. 
If the incident laser wavelength is shifted to higher wavelengths frequently fluorescence is lowered. Explanation from Edwards..
Using a NIR laser is favourable in removing fluorescence but on the other hand the intensity of the scattered radiation is lower due to dependence of inverse fourth power of the wavelength.[16, 22]
Fluorescence can be reduced if addition of a drop of water is added to the sample during analysis. Fluorescence quenching is caused in this case by heat regulation and hydrogen bonds ionisation of molecules.  The addition of water shift the Raman peaks. (bron)
Photobleaching is a light-induced change in a chromophore, resulting in the loss of its absorption of light of a particular wavelength.
Photobleaching is the irradiation of a sample with laser light for a longer period. Finally all electronic excited states are filled and fluorescence is reduced or nullified. When excitation is stopped all electrons will emit and fall down into their first energy state.  raman is faster than fluorescence.
A molecule of a fluorescent dye commonly used to label.. can withstand about 106 excitations by photons before light induced reactions destroy its pi system and the molecule stops to fluoresce. (atkins)
The investigation of dyes is rather difficult due to the minor differences in structural formula and therefore in the Raman spectra.
(dis)advantages of the use of Surface enhanced Raman scattering in Art research
Despite all the investigations in Raman technique some objects and groups of molecules need another method for analysis. Raman band intensities can be enhanced by application of metal colloids and hence the fluorescence signal will be lowered, Surface Enhanced Raman Scattering or Spectroscopy (SERS).
The largest advantage is that with less sample size or lower concentrations the signal is higher due to enhancement of the signal.
Also the synthesis of colloids is reasonably easy.
The biggest disadvantage of SERS is that it is an invasive technique and since the objects in cultural heritage should be handled very careful. So it should be weight what is more important knowing the sample or not knowing it but still complete. Raman is a micro destructive or non destructive technique but SERS is even less micro destructive, as can be seen that more and more research is done on SM SERS. (evt pas in discussion)
The expected enhancement with SERS is a little discussable but is somewhere between .... The synthesis ad application of colloids necessary for SERS is reasonably easy. The enhancement of signals using SERS is that high that smaller samples can be used and samples with too low concentrations for normal Raman are now analysable.
Raman: what is already done in art?
That Raman spectroscopy is a useful technique in cultural heritage research can be deduced from the many groups are studying this topic and the growing amount of articles published every year. [1-2, 16, 25-28] Within this subject research is done on pigments and dyes in archaeological objects, textile, paintings paper and many other kinds of artwork.
In the field of art research mainly pigments are analysed with Raman. This research is mainly focussed on small groups of pigments and libraries are difficult to find. Though one a pre 1850 pigment library and a flowchart of yellow pigments by Van den Abeele are available. [2, 16]
In archaeological artefacts many pigments were used e.g. in wall painting from the archaeological site Aurotiri on Thera Island (Greece). With Raman it was possible to determine the used pigment. [16, 20, 29-32]
Pigments in ink and also inks are examined with Raman. Naturally aged inks on parchment from the nineteenth century were successfully identified [25, 33-35]
Raman microscopy is used for identification of pigments on textile fibres. The use of Raman is easy because no sample preparation is needed, and it is able to identify colorants at the surface of micro samples. Research is done also on dyes on fibres [11, 37-38], with an extraction or directly on fibres themselves [39-41]
Analysis of components of complete cross sections of paint are also of great interest. Next to colorants also resins, binding media, or varnishes, generally organic molecules, are present. In contrast with FTIR it seems that Raman does not have problems with fluorescence of these components. 
As well there are studies done on the determination of these molecules/additives. Protein-based paint media show Raman bands but do not mask the dye bands though this depends on concentrations of both substances. This also applies for resins[20, 43], varnishes and binding media[42, 44].
SERS: what is already done in art?
Ook over pigments en art en sers.
Pigments are reasonable raman scatterers though a lot of research is done on pigments.
 Van Elslande used SERS next to conventional micro raman spectroscopy to investigate archeological pigments. For some pigments raman was sufficient, though SERS gives better-resolved spectra increases signal to noise ratio and reveal the presence of dyes incorporated in organic subtsrate
Nu weer dyes wat is het dye of pigments.
Voorlopig het enige meer!
Sers met anthraquinonen anthraquinonen zijn in dit article met nabh4 gedaan en komen geode resulaten uit.zoweel synthetische kleurstof als historische kleurstoffen [45-46]: Anthraquinone dyes are known for their fluorescence, Chen et al investigated them with sers. They used Tollens mirrors (bron) and Ag-AL2O3 substrate. It showed that it was sensitive and highly producible. Nice spectra were obtained with both methods. They also tried in situ sers, this proof of concept showed that it works and has to be extended to other dyes. Only for anthraquinone dyes this can be a problem because thy have already a metal ion bonded with fibres.
Crystal violet and sers poeh..
SERR :Xanthene and sers R6G proofed to be a good SERS probe and therefore used for much research.
Fibre met anthraquinone moet extraheert worden voor sers gebruikt omdat ze al gebonden zijn met een ander metaal.
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