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

Abstract

AbstractDecoupling 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 ~106) 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), CO 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. COO‐ stretch, COO− vs. NH3+ stretch) that coincides with the isoelectric pH of tyrosine. Finally, reliable quantification of the charge and concentration of tyrosine using SCs are established, enabling its use for studying intrinsic cellular pathways using SERS.