In Situ Two-Step Photoreduced SERS Materials for On-Chip Single-Molecule Spectroscopy with High Reproducibility

A method is developed to synthesize surface-enhanced Raman scattering (SERS) materials capable of single-molecule detection, integrated with a microfluidic system. Using a focused laser, silver nanoparticle aggregates as SERS monitors are fabricated in a microfluidic channel through photochemical reduction. After washing out the monitor, the aggregates are irradiated again by the same laser. This key step leads to full reduction of the residual reactants, which generates numerous small silver nanoparticles on the former nanoaggregates. Consequently, the enhancement ability of the SERS monitor is greatly boosted due to the emergence of new "hot spots." At the same time, the influence of the notorious "memory effect" in microfluidics is substantially suppressed due to the depletion of surface residues. Taking these advantages, two-step photoreduced SERS materials are able to detect different types of molecules with the concentration down to 10⁻¹³m. Based on a well-accepted bianalyte approach, it is proved that the detection limit reaches the single-molecule level. From a practical point of view, the detection reproducibility at different probing concentrations is also investigated. It is found that the effective single-molecule SERS measurements can be raised up to ≈50%. This microfluidic SERS with high reproducibility and ultrasensitivity will find promising applications in on-chip single-molecule spectroscopy.

On-chip single-molecule surface-enhanced Raman scattering (SERS) monitors free of the "memory effect" are fabricated in microfluidics by the two-step photoreduction method. Proved by bianalyte statistics, on-chip single-molecule detection is accomplished. This is a quick and well-reproducible microfluidic SERS technique with the detection limit as low as 10⁻¹³m. At the single-molecule level, the detection reproducibility can reach up to 50%.

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