DE102013008104A1 - SERS substrate - Google Patents

Abstract

The invention relates to SERS substrate with a carrier made of a carrier material which has a surface, and a substrate made of a substrate material with a dielectric function εM, which is arranged on the surface of the carrier and has a structure, the carrier material and the substrate material such are chosen so that a value X is at least 6, where X is calculated as follows: where the individual quantities represent the following: ωs: Stokes displacement = 1000 cm-1 ωE, d: excitation frequency at which the first factor of X becomes maximum, ωE , i: excitation frequency at which the second factor of X becomes maximum and εair: dielectric function of air = 1 εD: dielectric function of the carrier material at a wavelength λ = 600 nm

Description

The invention relates to an SERS substrate having a carrier made of a carrier material which has a surface, and a substrate made of a substrate material having a dielectric function ε _M , which is arranged on the surface of the carrier and has a structure.

SERS substrates have long been known in the art. In the field of analytics, Raman spectroscopy has become more and more established in recent years. In this case, the interaction of monochromatic radiation, in particular laser radiation, with molecular oscillations is utilized. This leads to characteristic, molecule-specific bands in the scattered light spectrum, which represent a fingerprint of the respective molecule. In this way, Raman spectroscopy allows a direct species-selective detection and thus provides a high information content about the respective molecular structure. However, the detection of a small number of molecules poses a great challenge since the Raman scattering process is naturally very faint. In the field of bioanalytics or forensic analysis, however, only very small amounts of a substance are often available for detection.

Therefore, efforts are being made to increase the sensitivity of the detection method. One possibility for this is the application of the so-called surface enhanced Raman scattering (SERS), in which the molecules to be detected are adsorbed on a nanostructured precious metal surface, resulting in a dramatic amplification of the Raman scattered light SERS substrates for surface-enhanced Raman spectroscopy are available on the market, with the SERS substrate comprising a support of a silicon material that is vapor-deposited with a thin layer of gold, and using such an SERS substrate as an intransparent support material Raman spectroscopy is performed only in reflection, which means that the excitation beam, which is used to excite the molecules to be detected, is passed through the air or an optically transparent solvent and then directly hits the substance to be determined the O surface of the SERS substrate has accumulated.

If a material which is transparent at the excitation frequency is used as the carrier material, Raman spectroscopy can also be carried out with an indirect beam guidance. This means that the excitation beam is passed through the carrier material and then impinges on the substance to be determined, which is adsorbed on the surface of the SERS substrate.

From the DE 10 2008 048 342 A1 For example, a method for producing such a SERS substrate is known. In this case, an initially applied noble metal layer which has been vapor-deposited onto the surface of the carrier material is exposed to pulsed laser radiation, so that nanoscale particles are formed which are responsible for a high SERS gain.

The lower the amount of a substance to be detected, the so-called analyte, the greater must be the amplification of the Raman scattered light. Therefore, different approaches are known from the prior art to further increase the sensitivity of surface-enhanced Raman spectroscopy.

From the WO 2012/015443 A1 For example, an optical fiber is known, at the end of which an SERS substrate is arranged. The fiber-optic cable has a flat end surface on which nanoscale field concentrators are initially applied, which are in the form of elevations or depressions. Only on these field concentrators, the actual substrate, such as a gold layer applied. As a result, the available surface is significantly increased. The molecules to be determined are deposited on this gold surface and can be identified by means of an SERS measurement. Although this increases the detection sensitivity, such a SERS substrate can only be produced with great difficulty because of the nanoscale field concentrators.

From the US 7,684,035 B2 In contrast, an SERS substrate is known in which a multiplicity of nanoparticles which consist of a metal and form the substrate are arranged on the carrier. These nanoparticles are at least partially embedded in a thin absorbent layer. The disadvantage, however, is that also in this method, a high manufacturing complexity required, since different materials on the order of a few nanometers must be arranged on the surface of the carrier. The layer thickness, in particular of the absorbing layer, has a strong influence on the Raman intensity, so that small deviations in the layer thickness can lead to strong effects.

Similar is from the US 2010/0085566 A1 known. Here, on the top side of the carrier, a multiplicity of nanopillars of a carrier material, for example silicon dioxide, are applied, which have a height of, for example, 50 nm. On top of this column a cover of a noble metal, for example silver, is applied, which has a thickness of a further 30 nm. This also results in an increased manufacturing expense, so that the production costs and the production time are increased.

The invention is therefore based on the object to present a SERS substrate that allows the best possible amplification of the Raman signal and yet easy, fast and inexpensive to produce.

wobei die einzelnen Größen folgendes darstellen:

ω_s:

Stokesverschiebung = 1000 cm^–1

ω_{E , d}:

Anregungsfrequenz, bei der der erste Faktor von X maximal wird,

ω_E,i:

Anregungsfrequenz, bei der der zweite Faktor von X maximal wird und

ε_Luft:

dielektrische Funktion von Luft = 1,

ε_D:

dielektrische Funktion des Trägermaterials bei einer Wellenlänge λ = 600 nm.

The invention achieves the stated object by means of an SERS substrate according to the preamble of claim 1, which is characterized in that the carrier material and the substrate material are selected such that a value X is at least 6, wherein X is calculated as follows:

where the individual quantities represent:

ω _s :

Stokes shift = 1000 cm ^-1

ω _{E , d} :

Excitation frequency at which the first factor of X becomes maximum,

ω _{E, i} :

Excitation frequency at which the second factor of X becomes maximum and

ε _air :

dielectric function of air = 1,

ε _D :

dielectric function of the carrier material at a wavelength λ = 600 nm.

The invention is based on the finding that the amplification of the SERS signal depends not only on the available surface and / or the geometric shape of the substrate and the underlying carrier, but in particular on the choice of materials used, ie in particular the carrier material and the substrate material. In addition, the invention is based on the finding that the amplification of the SERS signal can be further increased by indirect excitation, in which the excitation laser beam is thus conducted through the carrier material. Through the introduction of the size X, it is now possible to determine, essentially from the dielectric functions of the materials involved, which material combination of carrier material and substrate material is particularly suitable for producing a SERS substrate.

The actual amplification of the Raman signal depends on a variety of factors. In addition to the dielectric functions of the materials used, these also include, for example, the geometric design and shaping of the substrate material on the surface of the carrier. In order to be able to calculate this reinforcement exactly, the exact geometric structure of the substrate on the surface structure predetermined by the surface of the carrier would have to be determined and included in the calculation. This is associated with an immense amount of computation and with virtually unresolved difficulties and also, as the inventors have shown, unnecessary. Surprisingly, the gain can be calculated with a simple model, the result obtained, despite the in-depth approximations and assumptions, yielding good results.

The factor X and its calculation are based on this model. In the calculation, different quantities are assumed to be constant, which are known to be in fact not constant. Thus, the dielectric function of air ε _{air is set} to the value 1. For the dielectric function of the carrier material ε _D , the value is used, which has the dielectric function at a wavelength of λ = 600 nm. The Stokes shift ω _s is set as fixed at 1000 cm ^-1 in the calculation of the value X. For the dielectric function of the substrate ε _M , the wavelength-dependent values of the bulk material are used at wavelengths between 260 nm and 900 nm. This is described after the Drude model as:

in der v_F die Fermi-Geschwindigkeit und r den Radius des Partikels bezeichnet. Für die Fermi-Geschwindigkeit wird der Wert 1,4·10⁶ Meter pro Sekunde und für den Partikelradius der Wert 30 nm angesetzt. In this case, ω denotes the excitation frequency, which for the present model is assumed to be between 7.24 × 10 ¹⁵ radians per second and 2.09 × 10 ¹⁵ rads per second, which corresponds to wavelengths between 260 nm and 900 nm. ω _P denotes the plasma frequency, which is 1.35 x 10 ¹⁶ radians per second for silver and 1.4 x 10 ¹⁶ radians per second for gold. ω _D denotes a damping factor for silver at 7.62 x 10 ¹³ rads per second and for gold at 3.78 x 10 ¹³ rads per second. This damping factor takes into account that the substrate is a thin structured layer. It is calculated by the formula

in which v _F denotes the Fermi velocity and r the radius of the particle. For the Fermi speed, the value is 1.4 × 10 ⁶ meters per second and for the particle radius of 30 nm.

ε∞ is the high frequency dielectric constant limit, which is 2.48 for silver and 7.0 for gold.

Many of the assumptions and assumptions made are inaccurate for the situation to be described of a SERS substrate. For example, the substrate material is not applied in the form of a bulk material, but is present as a nano- or microstructured thin layer. That this layer can be described by the dielectric function of the bulk material according to the Drude model is surprising. In addition, it is believed that the dielectric function of the support material is substantially constant for the wavelength range of 260 nm to 900 nm for the respective materials used, and therefore can be set to a fixed value equal to the value of the dielectric function at a wavelength of 600 nm corresponds.

Surprisingly, the actual situation in a SERS substrate can obviously be described by this simplified model, which is why the factor X named in claim 1 offers a good and reliable measure for the selection of the different materials, despite the detailed approximations in its calculation.

The factor X is formed from the ratio of the signal amplification achieved by the surface-enhanced Raman spectroscopy with direct beam guidance (first factor) and with indirect beam guidance (second factor). Under direct beam guidance is understood that the excitation beam, which may be, for example, emitted laser radiation is passed through the air to the material adsorbed to the substrate to be detected. The Raman scattered radiation is also detected via this path. Therefore, the first factor of the value X depends only on the dielectric function of the substrate material and the dielectric function of the air. Herein lies another approximation of the model. Of course, even with direct beam guidance, the use of a medium other than air is conceivable. For example, it is known to carry out SERS measurements even with direct beam guidance through water. For the determination of the value X from the sketched model, however, only the comparison with air is used. In the present case, the first factor is understood to be the first expression of the parenthesis, while the term "second factor" refers to the second expression of the parenthesis.

Indirect beam guidance means that the excitation beam is passed through the carrier material of the carrier and then impinges on the material to be determined which is adsorbed on the substrate material. The Raman scattered radiation is also conducted through this path, that is, through the carrier material of the carrier, and then detected. Therefore, the second factor of the value X depends only on the dielectric function of the substrate material and the dielectric function of the substrate.

In the respective first factor in the two bracketed expressions, the dielectric function of the substrate material, which in particular is wavelength-dependent, is determined at the wavelength of the excitation frequency. This respective first factor in the first and the second parenthesis expression thus takes account of the incident excitation radiation which is irradiated at the excitation wavelength or the excitation frequency. The second factor in each case contains the dielectric function of the substrate material, which is determined at a frequency which corresponds to the Stokes shift of the Raman spectroscopy.

The Raman scattered radiation emitted by the Raman scattering on the material to be detected is shifted from the excitation radiation by a characteristic energy for the material and thus a corresponding wavelength. If the scattered radiation has a lower energy than the irradiated radiation, it is called Stokes radiation or Stokes bands in the Raman spectrum, while one speaks of anti-Stokes radiation or anti-Stokes bands in the Raman spectrum, when the scattered radiation has a higher energy than the irradiated radiation. The Stokes shift named here is intended to cover both effects. The Raman spectrum, which is recorded in surface-enhanced Raman spectroscopy, has peaks characteristic of the material. These occur at the respective Stokes shifts or Antistokesverschiebungen and are mostly known for the material to be detected.

An SERS substrate can be conventionally selected and manufactured for a very specific application. Thus, in particular, the material to be detected or the substance to be detected is known, so that the position of the individual Stokes shifts ω _{s is also} known. For the calculation of the value X, however, ω _{s is set} to 1000 cm ^-1 as a further approximation. The only variable that remains variable in the calculation method for X is the excitation frequency ω _E , which is now varied until the first factor of X and the second factor of X become maximum. In this way, the excitation frequencies ω _{E, d} for the first factor of X and ω _{E, i} for the second factor of X are determined. These excitation frequencies, at which the first factor or the second factor of X become maximum, are usually different from each other. They correspond to the excitation frequencies at which the desired peak of the selected Stokes shift is best recognized, because in this case the SERS signal gain is greatest.

The size X is now obtained by simply dividing the first factor and the second factor of X, so that a dimensionless quantity X is obtained.

Advantageously, the carrier material and the substrate material are selected such that X is greater than 7, preferably greater than 8, particularly preferably greater than 9. It has turned out to be particularly advantageous if the value X is greater than 10, preferably greater than 50, in particular preferably greater than 100.

For the determination of X, the maximum possible amplification of the Raman signal is therefore calculated by the surface-enhanced Raman spectroscopy by the variation of the respective excitation frequency for the direct and the indirect beam guidance. Subsequently, these two quantities, which at different excitation frequencies ω _{E, d} and ω _{E, i} ; can be divided, to get the value X. Thus, the maximum possible gain of the SERS signal is determined for a given Stokes shift of 1000 cm ^-1 . Advantageously, for the determination of ω _{E, d} and ω _{E, i} ; only a local maximum of the two factors can be determined, wherein only frequencies corresponding to a wavelength between 300 nm and 900 nm are used for the determination of this maximum in both the first factor and in the second factor. As a result, on the one hand, the computational effort is reduced because only one local and no longer a global maximum must be determined. In addition, the possible frequencies are limited to the typical range for excitation frequencies, so that the respective maximum is determined only in the frequency range useful for the particular application.

Advantageously, the substrate material is gold, silver, copper, aluminum, platinum, sodium, indium, zinc, cadmium, gallium or a combination of several of these elements. These materials are characterized by a complex refractive index. Obviously, the ratio of the real part to the imaginary part of the complex-valued refractive index is of great importance for the magnitude of the amplification of the SERS signal.

Advantageously, the substrate material has a nanostructure or a microstructure. This can be achieved in many different ways. Thus, it is known to first apply a layer of the respective substrate material and then to structure it, for example, by acting with a laser, in particular a pulsed laser. The structures achieved can range from a few nanometers to a few micrometers in diameter and have characteristic length scales. Alternatively, it is also possible to apply coatings which do not form a complete layer but form so-called island films on the surface of the support material. Here too, correspondingly structured surfaces can be achieved.

Advantageously, the support material is a sapphire, diamond, flint glass, lead glass, polycarbonate, polystyrene or a combination of these materials.

Preferably, the refractive index of the respective material is as high as possible.

Polycarbonate and polystyrene have the highest refractive index of about 1.6 among the transparent polymers. In combination with gold as a substrate material, the following values result for the different substrate materials: Refractive index ε _D X diamond 2.42 5.9 10.5 Titanium dioxide (anatase) 2.52 6.4 10.6 silicon carbide 2.67 7.1 10.5 Titanium dioxide (rutile) 3.1 9.6 9.7 Heavy flint glass (SF6) 1.81 3.3 7.6 tantalum pentoxide 2.1 4.4 9.5

The refractive indices used were determined in the range at 500 nm (tantalum pentoxide) and 587.6 nm and 589.3 nm, respectively.

Advantageously, the carrier material is arranged in the form of a layer on a base material, in particular on a quartz glass. The thickness of this layer can be between a few nanometers and a few monolayers on one side and a few micrometers on the other side. For the amplification of the SERS signal, the contact area between the substrate material and the carrier material is obviously important, whereas the thickness of the layer of the carrier material has only a minor influence. As a result, even expensive materials made of carrier material can be used, since only a relatively thin layer of these materials has to be used.

Preferably, the support material in this case is SiO _x , Ta ₂ O ₅ , TiO ₂ , HfO ₂ , Nb ₂ O ₅ , MgO, ZrO ₂ , Al ₂ O ₃ , Ga ₂ O ₃ or a combination of several of these materials. In the case of SiO _x , the "x" designates a stoichiometric factor by which, for example, the refractive index of the material can be influenced. For example, for x = 1, the refractive index is n = 1.7. The value X with gold as substrate material in this case is X = 6.7. A refractive index n = 1.8 gives X = 7.6, while a refractive index n = 1.9 gives a value X = 8.3. These calculations were carried out in each case with SiO _x as support material and gold as substrate material.

In this way it is possible, for example, to arrange the substrate directly on an end surface of the optical conductor, in particular of the glass fiber cable. This is particularly advantageous in medical technology, in which, for example, the examination of soft material, such as organs, is necessary. Often it is not possible or at least not intended to take a sample of the material to be examined, for example a human organ. When the SERS substrate is placed on the end of a fiber optic cable, for example, an organ may be penetrated as though with a needle and analyzed simultaneously. This gives spatially resolved information about the composition of the organ, which can be used in particular in medical diagnostics, for example to distinguish benign from malignant tumors. This represents a significant improvement over the prior art, which is often to be examined with a camera via a probe such as the esophagus, stomach or intestine. Even with these methods, although conspicuous sites can be located, a direct statement as to whether, for example, a benign or a malignant tumor is present, is not possible via a camera. In particular, in combination with a SERS substrate described here at the end of an optical conductor, such as a fiber optic cable, the prominent location can be examined right in place, so that the desired statements can be made quickly and safely. A removal of tissue samples is not necessary.

It has proved to be advantageous if the carrier is part of a wall of a reaction container or of a microfluid system. Within the reactor or the microfluidic system is a medium, for example a liquid, in which, for example, the concentration or even the presence of certain substances is to be detected. If the SERS substrate is integrated directly into the wall of the reactor, in which, for example, the carrier is part of the wall of the reaction vessel, excitation radiation can be conducted, for example, via an optical conductor directly to the inner wall of the reactor forming the surface of the carrier. In this way, an in-situ diagnosis and monitoring of the concentration of the material to be detected or the substance to be detected in the reactor vessel is possible.

The invention solves this problem by a set for performing an SERS measurement with a SERS substrate described herein and a laser for emitting laser radiation, which is used in particular as excitation radiation, and having a wavelength of ω _{E, i} ± 50 nm , preferably ± 20 nm, more preferably ± 10 nm. The excitation wavelength ω _{E, i} was found to be the optimum excitation wavelength to detect the desired scattered radiation shifted in frequency by the Stokes shift. It is therefore optimal if the laser radiation which can be emitted by the laser has exactly this frequency ω _{E, i} .

With the aid of a drawing, an embodiment of the present invention will be explained in more detail below. It shows:

1 - The schematic sectional view through a SERS substrate according to a first embodiment of the present invention.

1 shows a sectional view through an SERS substrate 1 that has a carrier 2 with a surface 4 features. The carrier 2 consists of a carrier material. On the surface 4 there is a substrate 6 from a substrate material. This substrate 6 has a micro or nanostructure that is in 1 not shown. At the the carrier 2 opposite side of the substrate 6 an analyte is deposited 8th which is to be detected by a SERS measurement. At the substrate 6 it is a plasmonic material, which may consist of the already named materials.

By a first arrow 10 a direct irradiation is shown. Laser radiation of the excitation frequency is applied to the analyte by the air or other medium used, for example water 8th sent and stimulates this. The analyte 8th then emits frequency-shifted electromagnetic radiation traveling in the opposite direction, in 1 So up, is detected. Electromagnetic radiation in the carrier 2 is therefore not used. In this type of direct irradiation often a carrier material is used, which is intransparent at the wavelengths of electromagnetic radiation used.

A second arrow 12 indicates the indirect direction of irradiation. The excitation radiation is transmitted through the carrier 2 and the substrate 6 on the analyte 8th sent and stimulates this. The of the analyte 8th emitted electromagnetic radiation is in the opposite way, in 1 So down, through the carrier material of the carrier 2 passed through and then detected. This may vary depending on the choice of materials used by the wearer 2 and the substrate 6 lead to a significant amplification of the SERS signal and thus to a significant increase in the sensitivity of the measurement.

LIST OF REFERENCE NUMBERS

1

SERS substrate

2

carrier

4

surface

6

substratum

8th

analyte

10

first arrow

12

second arrow

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102008048342 A1 [0005]
• WO 2012/015443 A1 [0007]
• US 7684035 B2 [0008]
• US 2010/0085566 A1 [0009]

Claims (10)

SERS substrate having a carrier of a carrier material which has a surface, and a substrate of a substrate material having a dielectric function ε _M , which is arranged on the surface of the carrier and has a structure, characterized in that the carrier material and the substrate material are chosen such that a value X is at least 6, where X is calculated as follows:

where the individual quantities represent: ω _s : Stokes shift = 1000 cm ^-1 ω _{E, d} : excitation frequency at which the first factor of X becomes maximum, ω _{E, i} : excitation frequency at which the second factor of X becomes maximum and ε _air : dielectric function of air = 1 ε _D : dielectric function of the carrier material at a wavelength λ = 600 nm SERS substrate according to claim 1, characterized in that the carrier material and the substrate material are selected such that X is greater than 10, preferably greater than 50, in particular preferably greater than 100. SERS substrate according to claim 1 or 2, characterized in that for the determination of ω _{E, d} and ω _{E, i} a local maximum of the first factor and the second factor is calculated for frequencies corresponding to wavelengths between 300 nm and 900 nm , SERS substrate according to one of the preceding claims, characterized in that the substrate material comprises gold, silver, copper, aluminum, platinum, sodium, indium, zinc, cadmium, gallium or a combination of several of these elements. SERS substrate according to one of the preceding claims, characterized in that the substrate material has a nanostructure or a microstructure. SERS substrate according to one of the preceding claims, characterized in that the support material is sapphire, diamond, flint glass, lead glass, polycarbonate, polystyrene or a combination of these materials. SERS substrate according to one of the preceding claims, characterized in that the carrier material is arranged in the form of a layer on a base material, in particular on quartz glass. SERS substrate according to claim 7, characterized in that the carrier material SiO _x , Ta ₂ O ₅ , TiO ₂ , HfO ₂ , Nb ₂ O ₅ , MgO, ZrO ₂ , Al ₂ O ₃ , Ga ₂ O ₃ or a combination of these Materials is. SERS substrate according to one of the preceding claims, characterized in that the carrier is part of a wall of a reaction vessel or a microfluidic system. Set for performing a SERS measurement with a SERS substrate according to any one of the preceding claims and a laser for emitting laser radiation having a wavelength of ± 50 nm, preferably ω _{E, i} ± 20 nm, more preferably ω _{E, i} ± 10 nm.

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