Composition of the SERS substrate
The active SERS area is formed using an ultra-short pulse laser on a soda-lime glass substrate. The substrate material is a weak Raman scattered and therefore particularly suitable for SERS (as compared to most crystalline materials). The resulting surface structure features stochastic nanopattern, which meets good resonance characteristics for various excitation wavelengths and adsorbed analyte molecules. A single SERS substrate can be used for various experimental conditions, analytes and results in a very high relative enhancement ratio of the Raman scattering up to 106. Silver-plated SERS works well not only in the IR range of excitation wavelengths, but also in a visible (blue, green) range as well.
Convenient size of the overall substrate (25 x 25 x 1 mm) is approximately a third part of a regular microscope slide, therefore it fits nicely into universal multi-wavelength Raman microscopes and can be used with dedicated compact SERS readers or spectrometers as well. Active area of the standard 'Randa' SERS substrate is 4 x 4 millimetres by default. Flexible manufacturing technique allows to change (increase or decrease) the size of the SERS area on request. Active areas as large as 10 x 10 mm have been produced specifically for application of electrodes (electro-chemical experiments).
All 'Randa' Raman substrates are vacuum packed in a cleanroom environment. No glue or other chemical substances are used during manufacturing, i.e. for attachment of active area chips to submounts (which is a common feature of counterpart products).
Opening of the package
‚Randa‘ SERS substrates are packed in microscope slide mailer boxes, containing 3 substrates in a single box. When opening a multi-substrate package, one has to think about possible contamination from outside atmosphere.
Make sure to open the substrate package in a clean-room environment. Whether there are more SERS substrates packed within a single slide mailer, it is necessary to keep the remaining items in a clean and dry atmosphere at all times before all substrates are consumed.
The stochastic nano-pattern on the active area of the substrate contains features from few nanometers to almost a micron in size. Analyte molecules, attached in the proximity of a certain combination of nano-features may experience the so called 'hot spot' effect, which leads to extremely high enhancement of Raman scattering.
By irradiating the active area with a laser beam, focused to a spot size of several to tens of micrometers, will cause appearance of the 'hot spots' within the perimeter of the focused beam. Therefore it is recommended to use focusing conditions to create laser focal spots as big as 20 microns or more. This will result in an averaging of the Raman signal from the hot-spots and surrounding nano-pattern.
SERS 'uncertainty principle' tells that a SERS substrate can have either a very high enhancement ratio or a good repeatability of measurements. 'Randa' SERS substrate is developed for highest sensitivity, therefore it is more suitable for trace detection instead of concentration measurement.
Test results usually show a deviation of peak amplitude from one test to another at different locations of the same SERS substrate. Deviation of up to 10% is very likely, therefore for concentration measurement it is advisable to calculate an average from few tests on the same active area. The relative enhancement ratio might differ from substrate to substrate, however current test results show good repeatability of sensors, i.e. the variation is more dependent on hot-spot allocation than on a production batch of the substrate.
Selection of the laser power
For the most accurate results it is highly recommended to choose laser power carefully. For Raman microscopy we recommend to start with 0.1mW but do not exceed 5mW. Depending on the objectives employed laser power should be set accordingly. For fiber optic Raman analyzers we recommend to use laser power in the range from 1mW to 50 mW.
There are three main ways to sample analyte, which is to be deposited on the active area. These are drop deposition, vapor deposition or immersion.
When immersion method is used, silver delamination from the edges of substrate is possible. In this happens, you can carry on with the experiment as it would not have influence in the final results since silver is on the active surface.
Always consider adsorption
Especially for small concentration detection, it is very important to take measures, which increase adsorption of desired molecules to the metal surface, otherwise the molecules may simply not interact with the surface nanostructures as the Raman signal rapidly decreases with increased distance from the surface. Thiolized compounds are often used to increase adsorption. Also, creation of electromagnetic field is sometimes used in order to bring charged particles closer to the surface, whereas the SERS substrate is used as one of the electrodes.
Term for utilization
It is recommended to utilize the substrates within 2 months from its production date, whereas longer storage durations may impact sensor performance. Also, depending on the environment in which the substrates are retained after unpacking, the recommended utilization time is within a day.