Exploring the Potentiality of a SERS-Active pH Nano-Biosensor

Abstract

The merging of the molecular specificity of Raman spectroscopy with the extraordinary optical properties of metallic nanoarchitectures is at the heart of Surface Enhanced Raman Spectroscopy (SERS), which in the last few decades proved its worth as powerful analytical tool with detection limits pushed to the single molecule recognition. Within this frame, SERS-based nanosensors for localized pH measurements have been developed and employed for a wide range of applications. Nevertheless, to improve the performances of such nanosensors, many key issues concerning their assembling, calibration and stability, that could significantly impact on the outcome of the pH measurements, need to be clarified. Here, we report on the detailed characterization of a case study SERS-active pH nanosensor, based on the conjugation of gold nanoparticles with the pH-sensitive molecular probe 4-mercaptobenzoic acid (4MBA). We analyzed and optimized all the aspects of the synthesis procedure and of the operating conditions to preserve the sensor stability and provide the highest responsiveness to pH. Exploiting the dependence of the SERS spectrum on the protonation degree of the carboxylic group at the edge of the 4MBA molecules, we derived a calibration curve for the nanosensor. The extrapolated working point, i.e., the pH value corresponding to the highest sensitivity, falls at pH 5.6, which corresponds to the pKa value of the molecule confined at the nanoparticle surface. A shift of the pKa of 4MBA, observed on the molecules confined at the nanostructured interface respect to the bulk counterpart, unveils the opportunity to assembly a SERS-based pH nanosensor with the ability to select its working point in the sensitivity region of interest, by acting on the nanostructured surface on which the molecular probe is confined. As a proof-of-concept, the nanosensor was successfully employed to measure the extracellular pH of normal and cancer cells, demonstrating the capability to discriminate between them.

DNA-functionalized gold nanoparticle assemblies for Surface Enhanced Raman Scattering

“…In the case of spherical AuNPs, 60 nm diameter, dispersed in solution, the peak is centered at λ = 535 nm. When the NPs are functionalized with a ligand molecule, as DNA, the dielectric environment changes and a redshift of the plasmonic peak is observed [36]. This is verified in the functionalization with both chain-A (∆λ ∼ 6 nm) and chain-B (∆λ ∼ 4 nm), as shown in Figure 2 a-b respectively, which ensures a successful and stable conjugation of the AuNPs with the DNA aptamers.…”

“…As the temperature decreases, the complementary DNA chains pair up, forming aggregates with increasing size. The observed widening, redshift and decrease of the plasmonic absorption peak intensity, along with the increased extinction in correspondence of the red-NIR spectral region (600 ÷ 800 nm), are expected consequences of the formation of large aggregates [36]. The temperature dependence of the observed behavior is a signature of the specificity of the DNA binding process, witnessing that the bridge mediated aggregation occurs by DNA hybridization into a double helix, which grants the NP assembly at controlled interparticle distance.…”

“…Conveniently, the SERS bands of DNA are outside the fingerprint region of common analytes detected by SERS spectroscopy [16,35,36]. In perspective, this will allow the use of this system as a robust and controlled sensing platform.…”

Dyes from the Ashes: Discovering and Characterizing Natural Dyes from Mineralized Textiles

“…The SERS approach is based on the contact between an analyte and noble metal nanostructures, which provides a great enhancement of Raman signals from electromagnetic and charge-transfer mechanisms [37]. The solution was analyzed in order to emphasize the signals of all the compounds extracted in a homogenous environment minimizing the effects related to different adsorption to the nanoparticles in the solid state, which could affect the SERS spectral shape and affect or hamper the molecular identification [38].…”

Assembling patchy plasmonic nanoparticles with aggregation-dependent antibacterial activity

“…They are inert and stable under most environmental conditions and exhibit low toxicity [14]. In addition, they allow for easy manipulation and surface conjugation, by both covalent and noncovalent interactions, with functional agents [16][17][18]. On top of this, AuNPs provide further antibacterial mechanisms, arising from their versatile optical and photothermal properties [13].…”