Combined near-infrared excited SEHRS and SERS spectra of pH sensors using silver nanostructures

Abstract

Surface-enhanced hyper-Raman scattering (SEHRS) and surface-enhanced Raman scattering (SERS) of para-mercaptobenzoic acid (pMBA) were studied with an excitation wavelength of 1064 nm, using different silver nanostructures as substrates for both SEHRS and SERS. The spectra acquired for different pH values between pH 2 and pH 12 were compared with SERS data obtained from the identical samples at 532 nm excitation. Comparison of the ratios of the enhancement factors from SEHRS and SERS experiments with those from calculations using plasmonic absorbance spectra suggests that the difference between total surface-enhancement factors of SEHRS and SERS for pMBA is mainly explained by a difference between the electromagnetic contributions for linear and non-linear SERS. SERS and SEHRS spectra obtained at near-infrared (NIR) excitation indicate an overall reduction of enhancement by a factor of 2-3 at very low and very high pH, compared to neutral pH. Our data provide evidence that different molecular vibrations and/or different adsorption species are probed in SERS and SEHRS, and that SEHRS is very sensitive to slight changes in the pMBA-nanostructure interactions. We conclude that the combination of SEHRS and SERS using NIR excitation is more powerful for micro-environmental pH sensing than one-photon spectra excited in the visible range alone.

SERS Nanotags and Their Applications in Biosensing and Bioimaging

“…Figure 2A-d shows the typical SERS spectra collected using 830 nm cw excitation (3 mW) in the endosomal compartments with different pH value. This study demonstrated pH imaging in single live cells at subendosomal resolution using SERS pH sensors is feasible [81]. Kneipp's group did a further research to monitor changes in local pH of the cellular compartments of living NIH/3T3 cells [87] and found that SERS nanosensors could dynamically monitor the change of local pH in individual live cells to be followed at subendosomal resolution in a timeline of cellular processes.…”

Surface enhanced hyper Raman scattering (SEHRS) and its applications

“…next section). 22,44 In the SERS and SEHRS spectra, the scattering powers P SERS and P SEHRS , respectively, are…”

“…Such estimations were made to, e.g., explain the origin of the difference between the SERS and SEHRS enhancement factors for non-resonant spectra of the dye crystal violet 44 and for para-mercaptobenzoic acid ( pMBA). 22 In analogy to the empirical relationship between the electromagnetic SERS enhancement factor and the surface plasmon absorption spectrum, …”

“…Different authors have reported typical spectral resolutions of 2-6 cm À1 in the full spectral range. 22,23,31,48,61 In experiments where the detection frequency range is identical in the one-and two-photon excited case, SERS and SEHRS spectra can be collected quasi-parallel in the same experimental set-up. 20,23,44,53,[62][63][64] Simultaneous detection of SEHRS and SERS when using the same excitation wavelength for both one-and two-photon SERS was also reported.…”

“…11,19,22,44,48 While in the 1980s, the reported SEHRS spectra were collected using double or triple monochromators with a photomultiplier or with intensified diode arrays, for examples see ref. 11, 19, 42 and 60, nowadays new generation deep-depletion CCDs, mainly N 2 cooled, back-illuminated, Si-based 2D cameras, are suitable for the relatively long integration times required, because of their extremely low dark noise and their high sensitivity in the whole wavelength range of interest.…”