Surface‐enhanced Raman scattering platform operating over wide pH range with minimal chemical enhancement effects: Test case of tyrosine

Abstract

Decoupling the contributions of electromagnetic and chemical enhancements in surface-enhanced Raman scattering (SERS) would lead to greater reliability and applicability in biological systems that involve large, localized pH gradients. However, suppressing short-range, chemical enhancement effects in SERS has proved challenging due to unavoidable charge-transfer interactions between analyte and the SERS platforms. This extends to both lithographically fabricated top-down and self-assembled bottom-up SERS platforms. Here, we employ a Soret effect-driven monodisperse nanoparticle assembly (Soret colloids [SCs]) with negligible surface charge (ξ < 16 mV) that provides exceptional SERS enhancement (analytical enhancement factor~10 6 ) contributed predominantly by excitation of localized surface plasmon resonance (electromagnetic enhancement) with minimal electrostatic or charge-transfer-based short-range interactions. Significantly, the low ξ of SCs is invariant over a wide pH range (pH 4-7) indicating minimal electrostatic surface charge. The interaction of such SCs with ionic analytes such as tyrosine leads to uniform SERS dictated by electromagnetic enhancement mechanism. The absence of contribution from chemical enhancement is clearly established by the vibrational fingerprints of tyrosine that are devoid of any spectral shifts originating from charge transfer between tyrosine and SCs. Accordingly, all features of the tyrosine that are independent of pH such as C-H stretch (2,700-3,000 cm −1 ) and C-C stretch (1,440 cm −1 ) and C-C-C bending (1,010 cm −1 ) are completely preserved without any spectral shifts. Simultaneously, pH-dependent vibrational features of tyrosine such as N-H stretch (1,545 cm −1 ), C¼O stretch (1,700 cm −1 ), COO − stretch (1,640 cm −1 ), C-O stretch (1,240 cm −1 ), and amide III bands (1,246 and 1,260 cm −1 ) are clearly observed. The intensities of such bands correspond exactly to the ionic state of tyrosine as dictated by the pH of the medium. This is reinforced by the crossover in the intensity of the pH-dependent vibrational modes (C-O vs. COOstretch, COO − vs. NH 3 + stretch) that coincides with the isoelectric pH of tyrosine. Finally, reliable quantification of the charge and concentration of tyrosine using SCs

Cellphone-Aided Attomolar Zinc Ion Detection Using Silkworm Protein-Based Nanointerface Engineering in a Plasmon-Coupled Dequenched Emission Platform

“…The “zone of inactivity” produced for fluorophores in <5 nm range from the Au surface is predominantly due to the coupling of fluorescent moieties to higher-order plasmonic modes that hasten the nonradiative pathways. − While this is the situation with bare AuNPs, recently, the so-called “quenching the quenched” phenomenon is demonstrated on account of plasmon intermixing or intercoupling in nanocavities, which eventually results in new superimposed modes that sustain improved radiative decay channels. − Consequently, the emitted photons are significantly increased at the far field from such nanogaps (near-field interfacial interaction). − Overall, this inadequacy of quenching encountered in experiments has been addressed using different approaches in the past few years and briefly listed as: (i) nanocavities generated with intense hotspots from nanogaps, (ii) metal–dielectric core–shell architectures, (iii) metal–dielectric-decorated nanointerfaces, and (iv) nanostructures with sharp tips and crevices (tip-core plasmons). − However, these methodologies present limitations of complex synthetic strategies (not-bioinspired) in nanofabrication with experimental artifacts apart from presenting moderate SPCE enhancements (<200-fold). Hence, the need for three-dimensional hotspots with heterometallic nanoassemblies has gained significant importance toward realizing dequenched and augmented SPCE enhancements.…”

“…27−31 However, these methodologies present limitations of complex synthetic strategies (not-bioinspired) in nanofabrication with experimental artifacts apart from presenting moderate SPCE enhancements (<200-fold). Hence, the need for three-dimensional hotspots 32 with heterometallic 33 nanoassemblies has gained significant importance toward realizing dequenched and augmented SPCE enhancements.…”

Xenobiotic Contamination of Water by Plastics and Pesticides Revealed through Real-Time, Ultrasensitive, and Reliable Surface-Enhanced Raman Scattering

“…Thus, overcoming this demands stable SERS platforms with a uniform density of intense electromagnetic hot spots. − Although nanoscale voids between nanoparticles are established to exhibit such intense electromagnetic hot spots arising from interparticle plasmon coupling, the ability to tailor and achieve monodisperse hot spots has been a persistent challenge. This bottleneck was recently addressed by demonstrating a thermally induced highly monodisperse assembly of metal nanoparticles termed as Soret colloids (SCs), for ultrasensitive and multiphasic detection of TNT, VOCs, and charged analytes. , Such nonlithographically assembled SCs present distinct advantages, such as ultrasensitive (∼10 3 counts/ppm), reliable, and quantifiable signals, over other SERS platforms. , Further, the predominant electromagnetic enhancement pathway observed in such SCs enables their operation over a wide pH range.…”

Plasmon-Coupled Directional Emission from Soluplus-Mediated AgAu Nanoparticles for Attomolar Sensing Using a Smartphone

“…This indicates that the emission is not only dequenched from AuNPs but was also significantly enhanced with sharp directionality and polarization, displaying unprecedented >1200-fold SPCE enhancements. The materials display exceptionally useful properties at nanoscale regimes as compared to their bulk counterparts. , The ultramodulated enhancement is attributed to a number of factors influencing the EM-field amplification resulting from (i) nanogaps between the adjacently located AgNPs, (ii) nanogaps between the adjacently located AuNPs, (iii) multifold nanogaps between the adjacently located AgAu nanohybirds in the ambient nanoenvironment of the thin film, (iv) nanogaps between AgNPs, AuNPs, AgAu nanohybrids, and the Ag thin film, (v) coherent plasmonic coupling between the adjacently located AuNPs, via the AgNP that is sandwiched in between. This enhanced plasmonic coupling is anticipated in our samples as the absorbance for AgAu nanohybrids is shifted to ∼570–580 nm (Figure a) from ∼530–540 nm (Figure a) as seen for AuNPs.…”