DE102007019166A1 - Process for the preparation of substrates for surface-enhanced Raman spectroscopy - Google Patents

Abstract

A process for producing a substrate for surface enhanced Raman spectroscopy comprising the steps of preparing a sol from a stabilized noble metal ion-containing solution and a precursor solution for a titanium oxide, coating a heat-resistant support by depositing the substrate Sols with a sol-gel process, pyrolysis and sintering of the layer with exclusion of light and illuminating the layer produced under exclusion of light at least on partial surfaces while heating at least the illuminated partial surfaces of the layer.

Description

The The invention relates to a process for the production of substrates for the surface-reinforced ("Surface Enhanced ") Raman spectroscopy.

Under the known method for the investigation of material properties Raman spectroscopy plays an important role. As Raman scattering refers to the inelastic scattering of light to matter (z. As crystals or molecules), in which a proportion of the irradiated Photon energy for excitation of lattice or molecular vibrations performs ("Stokes scattering" if the sample is present the light incidence was in the ground state). Also the reverse energy transfer and thus a blue shift of the scattered light ("anti-Stokes") occurs - albeit less frequently - The energy transfers are characteristic of the material and lead to defined wavelength shifts of the Raman-scattered Light.

Based Raman spectra in particular is the identification of molecules possible in low concentration in a solution occurrence. This is basically Raman spectroscopy a very robust process for chemical analysis in about the Biotechnology, pharmacy ("drug discovery"), medical Diagnosis or in environmental monitoring.

However, the classical Raman signal is very weak, because only less than 0.001% of the incident light is Raman scattered. But already described in 1974 Fleischmann and McQuillan (Chem. Phys. Lett. 26 (1974) 123) the dramatic increase in signal yield in the analysis of pyridine on a rough silver electrode as a substrate. In later work, it was confirmed that even the presence of a metal surface in the vicinity of a molecule to be excited to vibrate is sufficient to amplify the Raman signal by several orders of magnitude. The reason lies in the possibility of resonant excitation of plasmon in the metal, which then leads via electric field interaction (dipole-dipole) in turn to the enhanced excitation of molecular vibrations. It has proven to be advantageous if local electric field peaks can be formed ("hot spots"), so that no smooth metal surfaces, but preferably rough surfaces, are used.Theoretically, signal intensifications of up to 10 ^{12 can be} achieved ("Surface Enhanced Raman"). - SER) Reinforcements of about 10 ^{6 are} common on today commercially available SER substrates.

Next rough surfaces are also smooth, non-metallic surfaces with isolated metal tips (eg adsorbed clusters) suitable, to cause a SER gain. prerequisite for the SER effect is also here that the metal tips Plasmon resonance in the wavelength range of visible Light (VIS) to infrared (IR) show, with common Laser can be stimulated. This is especially true for precious metals Gold and silver are the case, which also prefers as a nanoparticle material used to make SER-active substrates.

Many known processes for the preparation of SER substrates pursue the application of metal colloids to non-metallic substrates, e.g. As semiconductor or glass, such as by dropping and drying of colloid suspensions, by laser deposition or by vapor deposition of metal under vacuum on a masked or shadowed substrate. The aim is to produce evenly spaced, over the surface of the substrate uniformly distributed metal particles in a regular arrangement. From the literature is known ( Lidong Qi et al. PNAS 103 (6), 2006, pp. 13300-13303 ) that particle diameters around 120 nm and mean particle spacings around 30 nm provide the best SER gains.

When an example of a masking that is particularly suitable for SER substrates seems to be the nanosphere lithography (NSL) mentioned. In the process, balls with several 100 nanometers are used Diameter on the substrate if possible in a monolayer arranged with tight ball packing. Thereafter, metal is evaporated and the balls are removed. In the interstices have hereafter roughly triangular, regularly arranged Metal islands that act as metal tips for SER can.

Naturally is the use of templates with structure sizes always particularly expensive on the nanometer scale. Specifically in the NSL exists z. As the problem, the formation of multiple layers of balls to prevent.

• – Günstig produzierbar in großer Anzahl;
• – Hervorragend reproduzierbar hinsichtlich SER-Signalverstärkung;
• – Chemisch inert bzw. robust gegen aggressive Chemikalien
• – Arrays von SER-Substraten sollten mit geringem Aufwand herstellbar sein.

In order to further develop SER spectroscopy into a widely used standard of trace analysis, substrates with the following properties should be available:

• - Conveniently produced in large numbers;
• - Excellent reproducibility in terms of SER signal amplification;
• - Chemically inert or robust against aggressive chemicals
• - Arrays of SER substrates should be produced with little effort.

The Requirement of a low-cost production leaves currently all template methods, at least for mass production not promising. Rather, one will deal with the Create irregularly arranged, but still at least statistically evenly distributed metal particles on a non-metal content to limit the effort.

The publication WO 2006/060734 A2 pursues this approach and proposes that the metal particles first grow in situ on the substrate - namely from metal ions that are located in a matrix forming the substrate from the outset. For this purpose, the metal ions (eg gold) are added to a precursor solution for a suitable oxide (eg silicon dioxide), which is then divided into equal portions and dried at ambient temperature for 12 to 48 hours. These pellets are treated with a reducing agent in two steps, whereby first the metal ions are rapidly reduced to form particle growth nuclei on the pellet surface, so that in the subsequent weaker reduction phase (duration: up to 12 hours) particles grow gradually on these nuclei. The WO 2006/060734 A2 attaches great importance to the fact that approximately equal-sized ("monodisperse-sized") metal particles are formed, which are located on the surface of the matrix substrate and are not embedded in it.

The described method of WO 2006/060734 A2 is clearly too time-consuming, not least because of the relatively slow treatment with the reducing agent, which is also still to dispose of.

It It is an object of the invention to provide a method which has a fast, reproducible and cost-effective production of SER substrates.

The Task is solved by a method with the features of the claim 1. The dependent claims give advantageous Embodiments of the invention.

The inventive method is initially based on the teaching of WO 2006/060734 A2 in that in this case as well, an additional noble-metal-ion-containing precursor is used for the preparation of a metal oxide film via a sol-gel process. The precursor, however, must contain titanium necessary for the realization of the invention in order to be able to form a titanium oxide matrix, preferably TiO ₂ .

When Precious metal ions are very particularly preferably used silver ions. Although other precious metals should not be excluded, but there are no studies available for this.

The invention now unexpectedly continues the known process by first subjecting the silver-ion-containing titanium precursor to a heat-resistant substrate (eg glass, semiconductor, metal) using a sol-gel process (in particular spinning, spraying, Diving) is applied there and immediately pyrolyzed with exclusion of light and sintered. After the heat treatment, the layer is dry and hard and largely resistant to chemical attack. It has virtually no silver nanoparticles on the surface and is therefore not suitable as a SER substrate. Another processing according to the doctrine of WO 2006/060734 A2 is also not suitable to improve this. The layer is very easy to store when stored in the dark.

If the silver-containing titanium oxide layer is now intensively illuminated with sufficient heating, electron-hole pairs are generated in the matrix according to FIG

Present silver ions have a strong tendency to absorb the released electrons. Ag ⁺ + 1e ^- → Ag - metal

It smallest particles of silver form, which are heated in the same time Matrix have a certain mobility. You can by diffusion also to larger particles connect. Silver in the immediate vicinity of the layer surface passes through them and forms silver nanoparticles on the Surface. Cools the now resulting silver-titania nanocomposite back to ambient temperature, the silver particle distribution becomes practically "frozen" with light and warmth treated, originally SER-inactive layer is suitable then as SER substrate, showing a gain factor, which compete with commercially available substrates can.

The The invention is explained in more detail below with reference to FIG an embodiment and to the associated Characters. Showing:

1 a scanning electron micrograph of the titanium oxide layer, which has the highest measured SER activity, with the silver particles thereon;

2 the results of two Raman measurements of a known dye (R6G), once on the substrate according to the invention (upper curve) and on the not yet exposed, with exclusion of light pyrolyzed and sintered substrate.

It follows an embodiment, which refers to the figures takes.

First, a TiO ₂ -Ag precursor solution (sol) is prepared. The silver content should be between 10% and 60% by weight based on the total mass of the (after pyrolysis and sintering) dried layer. Preferably, the Ag mass fraction is established between 30% and 60%. In the following example, it is about 50%, which is considered to be particularly advantageous.

• Ti-iso. = Ti-isopropoxid
• Hacac = Acetylaceton

For the preparation of 100 ml of an approximately 0.6 molar solution, 10 ml of 2-methoxyethanol and acetylacetone are initially charged in a beaker. Then the Ti isopropoxide is added, followed by stirring for 30 minutes. As a second solution, mix 10 ml of 2-methoxyethanol with water. After stirring for 30 minutes, the hydrous solution is added to the Ti-acetylacetone complex. Let it stir again for 30 minutes. For the silver solution, 10 ml of 2-methoxyethanol are placed in a beaker and AgNO ₃ and pyridine (as stabilizer) are added. This complex must also be stirred for 30 minutes. Thereafter, the silver solution may be added to the stabilized and hydrolyzed titanium solution. After stirring again for 30 minutes, 2 g of polyethylene glycol 400 are added to the solution, made up to 100 ml with 2-methoxyethanol and then filtered. The polyethylene glycol serves to crack-free layer formation. The exact weights, including mass, volume or molar information can be found in the following table. product reactant Vtot, ml mol G ml Molarity: 0.6 TiO2-50% Ag TiIV Propoxide Ti (OCHMet2) 4 100 0.0345 9.82767 10.23716 Ti: 57.5 at% AgNO3 - 0.0255 4.33245 0 Ti: 50.05557 wt% methoxyethanol - 0.68637 52.23245 54.18304 3 × 15.35186 + 8.12746 ml H2O - 0.138 2.48400 2,484 - acetylacetone - 0.01725 1.72707 1.67526 1.67526 ml for TiIVPr pyridine - .3825 30.25575 29.65063 29.65063 ml for AgNO3 PEG400 - - 2.00000 1.76991 -

• Ti-iso. = Ti isopropoxide
• Hacac = acetylacetone

Stoichiometries used:

• Ti-isoprop: Hacac: H ₂ O = 1: 0.5: 4 (mol)
• AgNO ₃ : pyridine = 1:15 (mol)

The TiO ₂ -Ag layers are produced by spin coating. The carrier used is exemplified oxidized silicon. The pyrolysis of the layers is then carried out at 250 ° C. The final treatment temperature (sintering step) is between 450 and 550 ° C. The thickness of the layers can be between 50 and 1000 nm. In the case of thermal treatment, it is essential to ensure that it is carried out in the absence of light (UV to the end of VIS). Only then is any uncontrolled silver precipitation / reduction ver avoided.

After production, the substrates can be exposed immediately or, if necessary, stored in the dark. The exposure can be done with virtually all conventional lamps that emit light in the spectral range of 280 to 800 nm. The TiO ₂ matrix absorbs very well over the entire visible range, which is why TiO _{2 is} also known as a solar absorber. If the lamp emits heat at the same time which heats the layer, SER activation can already be initiated by forming the silver particles on the layer surface. Alternatively, lower power light sources (eg, laser or light emitting diodes) may be used in combination with a heat source that allows a temperature of up to 250 ° C.

The Adjustment of particle size and distribution takes place through the interaction of exposure and heat. The Exposure leads to the reduction of silver from silver oxide in elemental silver, the heat for coarsening the particles by diffusion.

The Heat input should be set up for this purpose that the layer during lighting at least temperatures above 80 ° C. Preferable are temperatures between about 150 ° C and 250 ° C. It has become meanwhile proved not to be beneficial during the lighting Temperatures above 250 ° C to use, because then the mobility of the silver particles would be too big. It could be quite uneven Set particle distributions on the surface of the coating, which affect the SER activity.

Typical parameters for obtaining a substrate suitable for SER spectroscopy are e.g. B.
Green light (550 nm) at 150 ° C. Exposure time 1 hour
UV light (300 to 350 nm) at 200 ° C. Exposure time 30 minutes.

In the illuminated area of the layer, finely distributed silver particles are formed, the sizes and (average) distances of which depend on the treatment time. 1 shows the scanning electron micrograph of a layer which is illuminated for one hour with a halogen lamp, power consumption 1000 W, from close range. The surface temperature reaches up to 250 ° C. It is easy to see that a relatively broad particle size distribution (diameter about 100 ± 40 nm) sets in, but that occurs in almost the same way everywhere on the substrate. Likewise, the mean distances between the particles are relatively small and almost the same across the surface. It should be emphasized that the SER substrates produced here are anything but "monodisperse-sized" in the sense of WO 2006/060734 A2 which is not surprising given the significant differences between the methods.

The SER activity of the layer 1 can be evaluated with an argon Raman spectrometer at a wavelength of 514.5 nm. A color molecule (R6G, 10 ^-6 mol) whose Raman lines are known is analyzed to yield the 2 in which the Raman shift is given as a change in the wavenumber with respect to the incident radiation (k = 2π / λ = 122122 cm ^{-1 )} . The dashed lines show the known line positions. The lower trace is recorded on the still unexposed, silver-ion-containing titanium oxide and shows virtually no Raman signal. The upper curve emerges on the layer 1 and clearly shows the Raman spectrum of the sample. The amplification factors are in the range of 5 × 10 ⁶ for the substrate 1 and thus reach maximum values achievable for large-area Raman spectroscopy.

Because the unexposed, light-excluded substrate with the film on it no significant SER activity It is further proposed to provide an array of SER-active spots to produce on this film by the inventive Treatment with light and heat by means of an optical Perform mask. In principle, every light is absorbing or reflective masking suitable (depending on the Light source). In the simplest case, you can do a simple, mechanical Place template with recesses on the substrate you are removed after the exposure. When very small feature sizes can be desired, you can on known masking techniques z. B. from semiconductor technology, the be provided with a microstructure. The masking layer can after processing z. B. be removed chemically. she can but may also remain on the substrate if they are not contributes to SER amplification. The editing all separate spots (non-masked faces of the Substrate) is preferably carried out simultaneously (if necessary, light by means of lenses fanned), so that the processing parameters are identical are.

As expected, an array made in this way shows very good agreement of the properties of different spots. Of course, each array can also be cut into individual sample carriers. There the sol-gel technique is designed for large area coating anyway, mass production of SER substrates is likely to result in making relatively large SER spot arrays and cutting them as needed.

• • Es kommen nur industrielle Standardprozesse (Sol-Gel-Beschichtung, Pyrolyse, Maskierung, Belichtung) zum Einsatz, die mit entsprechend hoher Geschwindigkeit durchführbar sind.
• • Titanoxid-Schichten kommen vielfach zum Einsatz und sind als chemisch stabil bekannt. Entsorgungskanäle für gebrauchte SER-Substrate sind somit auch bereits vorhanden.
• • Abgesehen vom unvermeidlichen Ausbrennen der Organik bei der Pyrolyse werden keine weiteren Chemikalien benutzt und freigesetzt, d. h. es entsteht kein neues Entsorgungsproblem.
• • Die Reproduzierbarkeit der SER-Substrate ist – wie immer – eine Frage der präzisen Prozesskontrolle. Die hier eingesetzten Prozesse werden von der Industrie ausnahmslos beherrscht und erfordern keine neuen Entwicklungen.

In summary, the method according to the invention has the following advantages over the prior art:

• • Only standard industrial processes (sol-gel coating, pyrolysis, masking, exposure) are used, which can be carried out at correspondingly high speeds.
• • Titanium oxide layers are widely used and are known to be chemically stable. Disposal channels for used SER substrates are thus already available.
• • Apart from the inevitable burning out of the organics during pyrolysis, no other chemicals are used and released, ie no new disposal problem arises.
• • The reproducibility of the SER substrates is - as always - a matter of precise process control. The processes used here are all mastered by the industry and do not require any new developments.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - WO 2006/060734 A2 [0011, 0011, 0012, 0015, 0017, 0032]

Cited non-patent literature

• Fleischmann and McQuillan (Chem. Phys. Lett 26 (1974) 123) [0004]
• - Lidong Qi et al. PNAS 103 (6), 2006, pp. 13300-13303 [0006]

Claims (8)

Process for the preparation of a substrate for the surface-reinforced ("surface enhanced ") Raman spectroscopy with the steps: i. Produce a sol of a stabilized, noble metal ion-containing Solution and a precursor solution for a titanium oxide, ii. Coating a heat-resistant Support by applying the sol with a sol-gel method iii. Pyrolysis and sintering of the layer in the absence of light iv. Illuminate the layer produced under exclusion of light at least on partial surfaces with simultaneous heating at least the illuminated partial surfaces of the layer. Method according to claim 1, characterized in that that illuminating the layer made under exclusion of light on a plurality of spatially separated subareas takes place simultaneously. The method of claim 2, that under the exclusion of light prepared layer before lighting with an opaque Template that has recesses, is partially covered. Method according to one of the preceding claims, characterized in that used as noble metal ions silver ions be, where the silver mass fraction based on the mass of dried layer between 10% and 60% is set up. Method according to claim 4, characterized in that that the silver ion-containing solution by addition of Pyridine is stabilized. Method according to one of the preceding claims, characterized in that the pyrolysis to the exclusion of Light at temperatures around 250 ° C and sintering under exclusion of light at temperatures between 450 ° C and 550 ° C he follows. Method according to one of the preceding claims, characterized in that between the pyrolysis and sintering in the absence of light and lighting under exclusion of light prepared layer provided a storage time with exclusion of light becomes. Method according to one of the preceding claims, characterized in that the lighting of the under light prepared layer with heating at least the illuminated Partial surfaces at temperatures between 80 ° C and 250 ° C takes place.