SERS-based pH-Dependent detection of sulfites in wine by hydrogel nanocomposites

Sulfur dioxide (SO₂) and sulfites are well-known additives in winemaking due to their preservative properties. Although they can prevent oxidation and inhibit microbial growth, they pose health risks and require limitations on their use. Consequently, the total level of SO₂ is regulated and several quantification strategies have been proposed. The approved detection methods require the extraction of SO₂ by heating and/or acid treatment. Then, iodine or acid/base titrations are conducted for the detection of liberated SO₂. Although these methods can provide sensitive detection of SO₂, they are complex, time-consuming, and require sample preparation steps and skilled operators. Thus, to overcome these disadvantages, an easy-to-use method, involving simple sample preparation steps, and offering high sensitivity and selectivity, is desirable. Herein, we introduce a SERS-based strategy for SO₂ detection in liquids using hydrogel nanocomposites. The hydrogels are prepared by poly(ethylene glycol) diacrylate (PEGDA) in the presence of gold nanoparticles (AuNPs), acting as the SERS substrate. The use of hydrogels ensures a homogenous signal distribution and an efficient collection of SO₂, and drying the hydrogels enhances and stabilizes the obtained SO₂ signal. The detection strategy is based on the pH-dependent dissociation of SO₂. By adjusting the pH value of wine to 10 through simple dilutions, SO₂ can be directly detected in wine, down to 0.4 ppm, well below the regulatory limits. The proposed method allows for sensitive, direct, cost-effective detection of SO₂ by eliminating the loss of the gaseous form of the sample and avoids titration-based detection methods.