Integrating-sphere combined with SERS colloid for improved reproducibility in liquid-phase SERS quantification.
Abstract
Although surface-enhanced Raman spectroscopy (SERS) offers single-molecule sensitivity, its direct liquid-phase quantification has long been compromised by signal distortions caused by non-uniform enhancement and stochastic molecular diffusion. Conventional internal-standard or drying-based strategies are both tedious and unsuitable for real-time in situ analysis. To tackle this challenge, we incorporate an integrating sphere into the SERS system. Its high collection efficiency and spatially ensemble-averaged excitation and collection effectively average local field variations over the probed volume, transforming discrete hot-spot signals into a robust and reproducible ensemble response rather than relying on a strictly uniform irradiance. This design achieves, for the first time, direct quantitative SERS detection in a liquid dynamic environment, and crucially, bridges the gap between SERS fingerprinting and accurate metrology. The system achieves excellent detection performance for Rhodamine 6G (R6G) in water and pigments in different beverages, demonstrating a remarkable detection limit of 0.54 pM (≈2.59 × 10-4 ppb) (R6G), 0.44 ppb (allura red), 0.90 ppb (sunset yellow), 2.1 ppb (carmine) and 7.2 ppb (lemon yellow). The relative standard deviation (RSD) remained ≤ 7.39% (R6G) and ≤ 3.71% (pigments) at all concentrations, demonstrating excellent uniformity and stability. The dynamic detection range of the pigments were from 1 ppm to 0.5 ppb (allura red), 1 ppm - 1 ppb (sunset yellow),1 ppm - 6 ppb (carmine) and 1 ppm - 5 ppb (lemon yellow). The pigment contents in commercially available beverage samples measured by SERS (2.21, 11.03, 2.36, 1.67 ppm) and HPLC (2.30, 10.93, 2.41, 1.49 ppm) were very close, further demonstrating the quantitative accuracy of our method. The developed platform holds great promise as a versatile optical tool for quantitative detection of real water samples.
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