Advances in Hydrogel-Integrated SERS Platforms: Innovations, Applications, Challenges, and Future Prospects in Food Safety Detection

Abstract

Background: Food safety remains a global concern due to biological and chemical contaminants, including adulterants, pathogens, antibiotic residues, and pesticides. Traditional detection methods are accurate but limited by time requirements, complex sample preparation, high costs, and poor field applicability. Surface-Enhanced Raman Spectroscopy (SERS) offers non-destructive analysis with low detection limits and high specificity, yet conventional SERS substrates face challenges with reproducibility, nanoparticle aggregation, and sensitivity in food matrices. Hydrogels have emerged as supporting materials for SERS due to their water content, tunable porosity, flexibility, and ability to entrap plasmonic nanostructures. Scope and Approach: This review examines recent advances in hydrogel-integrated SERS platforms for food safety applications. The three-dimensional structure of hydrogels enables homogeneous distribution of metal nanoparticles, prevents aggregation, and offers analyte enrichment. We analyze material design, functionalization strategies, and how hydrogel properties-crosslinking density, porosity, surface charge, and nanoparticle distribution-influence SERS performance in food matrices. Key Findings and Conclusions: Hydrogel-integrated SERS platforms demonstrate superior performance in detecting various food contaminants-including pesticides, adulterants, and additives-in real food matrices, often achieving detection limits in the nanomolar to picomolar range, depending on the analyte and substrate design. Current limitations include storage stability concerns, batch-to-batch variability, and regulatory acceptance hurdles. Future research directions should focus on multiplex detection capabilities, integration with smart sensing technologies, and industrial scalability to facilitate practical deployment in global food safety monitoring across diverse supply chains.

Keywords: food safety detection; hydrogel; nanoparticle stabilization; smart materials; surface-enhanced Raman scattering (SERS).

Figure 2

Natural-material-based hydrogels for SERS food safety applications. (A) Alginate hydrogels: (a) Gelation mechanisms of alginate via ionic interactions with divalent cations, polyelectrolytes, and acids, forming tunable 3D networks; (b) Chemical structure of alginate and formation of “egg-box” model through Ca²⁺ crosslinking of α-L-guluronic acid (G) blocks; (B) Cellulose-based hydrogels: (a,b) Molecular structures of cellulose and its polysaccharide chains; (c) Schematic comparison of natural and treated elastic wood: structural recovery and water mobility due to cellulose network integrity, enabling reversible shape change and water responsiveness; (C) Chitosan hydrogels: (a) Sources of chitin from crustaceans and fungi; (b,c) Conversion of chitin (water-insoluble) to chitosan (water-soluble) via deacetylation; (d,e) Hydrogel formation via acetylation and sol–gel transformation; (f–h) Crosslinking strategies and intermolecular interactions enabling chitosan’s gel structure and analyte-binding capabilities.; adapted from [71,72,73,74,75,76,77].

Figure 3

Structural parameters influencing the function and SERS performance of hydrogel systems in food safety detection. (A) Crosslinking density: Illustration of crosslink density (ρ_x) as a composite of ionic (ρ_x,ion) and hydrogen (ρ_xhyd) interactions, determined through rheology and low-field NMR (LF-NMR). Adjustments in polymer–crosslinker ratios influence network stiffness and permeability, which are critical for analyte diffusion and SERS enhancement. Adapted from [160]; (B) Porosity: (i) Synthesis of a Ag/PNIP-LAP hydrogel through UV initiation; (ii) The hydrogel exhibits responsive porosity, expanding to absorb small hydrophilic molecules and contracting against macromolecular interference, providing adaptive sieving and enhanced SERS signal for food contaminants. Adapted from [174]. (C) Surface charge: Electrostatic trapping at the water-hydrogel interface using positively charged PVA–Ag hydrogel (CYH). A1–A4: Charged pesticides migrate and accumulate at the interface due to electrostatic attraction. B1–B4: Optical images show self-assembly progression, improving hotspot density and Raman signal under laser excitation. Adapted from [41]; (D) Hydrophobicity: Double-network hydrogel containing polyacrylamide, Na-MMT, and hydrophobic domains structured via an “egg-box” architecture. The matrix selectively traps hydrophobic molecules like λ-cyhalothrin, reducing release rates and increasing SERS sensitivity in lipid-rich food matrices. Adapted from [192].

Figure 4

Functional advantages of hydrogel-integrated SERS platforms for enhanced food safety detection. (A) Schematic of the SERS effect: laser excitation induces localized surface plasmon resonance between plasmonic nanoparticles, generating electromagnetic “hot spots” that amplify Raman signals from analytes [212]; (B) A hydrogel-based SERS chip developed with AgNPs-PVA for T-2 toxin detection: cyclic freeze-thawing enables the formation of a stable matrix, allowing for the sensitive detection of toxins in grain extracts with a portable Raman spectrometer [217]; (C) (a) Fabrication of flexible AgNP–BNC (bacterial nanocellulose) composite paper via vacuum filtration; integration of AgNPs into BNC film ensures stable, sensitive SERS response; (b) (i–v) SEM images showing top view, bottom view, and cross-sections of AgNP–BNC paper structure, highlighting embedded AgNPs and uniform distribution for high SERS activity; (c) (i) Schematic of in situ detection mechanism; (ii,iii) Raman spectra showing sensitive detection of methomyl pesticide on real fruit surfaces. [218]; (D) Flexible microfluidic paper-based SERS sensor fabrication: (a–f) Step-by-step process from wax-coated paper to heat-treated hydrophilic wells incorporating Ag-decorated graphene oxide for ultra-sensitive analyte capture and signal enhancement. Final spectra show successful detection of contaminants with high reproducibility and long-term stability [221].

Figure 5

Hydrogel-integrated SERS platforms for detecting food adulteration and fraud. (A) Schematic of a label-free SERS detection system using Au@AgNP-MBN-loaded PEGDA hydrogel micropellets. Droplet generation followed by UV crosslinking creates porous pellets enabling sensitive detection of melamine in milk without pretreatment [226]; (B) DNAzyme-functionalized hydrogel SERS chip for detecting uranyl ions in fish. The smart hydrogel capsule undergoes conformational change upon DNA–ion interaction, triggering a switchable SERS signal. The process involves selective analyte capture, signal on/off switching, and real sample testing on fish (highlighted area) [227]; (C) Fabrication of a light-trapping hydrogel SERS substrate inspired by compound eyes; (i) Self-assembly of Ag(NH₃)₂⁺ nanostructures into periodic arrays via nanosphere lithography and ion-assisted deposition; (ii–vii) SEM images at different magnifications show uniform nanostructures critical for enhancing SERS signal via light confinement and plasmonic coupling [228]; (D) Cicada-wing-inspired nanostructured substrate for melamine detection; (a) Image of a cicada and its wing nanostructures (i: macroscopic; ii: AFM of surface); (b) Fabrication of Ag-coated cicada-structured replica glass (Ag TS-CSRG) using UV-curable epoxy; (c–e) Schematic renderings of surface topographies used to replicate and optimize light trapping and SERS enhancement [98].

Figure 6

Hydrogel-integrated SERS platforms for rapid and sensitive pesticide residue detection in food samples. (A) SERS platform based on a PNIPAM/PEGDA/PVP hydrogel composite embedded with AgNPs ((a) Surface morphology of PNIPAM hydrogel; (b) SEM image of AgNP-loaded PNIPAM composite; (c) Raman spectra showing SERS activity of the composite hydrogel; (d) Elemental mapping confirming Ag presence; (e,f) Visual demonstration of hydrogel patch on apple for direct pesticide detection; (g–i) Flexibility of hydrogel membrane enabling surface adaptability [246]); (B) Sodium alginate hydrogel embedded with Au@Ag nanoparticles for thiram detection in juice samples ((i) Working principle of target-specific ion crosslinking and selective SERS sensing using Ca²⁺ and PBS washing; (ii) Workflow showing sample preparation from squeezed fruit juices and direct detection; (iii) Raman spectra for thiram at different concentrations (10⁻¹⁰–10⁻⁴ M); (iv) Calibration curve demonstrating excellent linearity and sensitivity [143]); (C) Hydrogel-based SERS platform for detecting maleic hydrazide using a-AgNPs in a 96-well format. Soaking and 785 nm laser irradiation generate strong electromagnetic (EM) enhancement and localized surface plasmon resonance (LSPR), enabling high SERS signal readout [49]; (D) Fabrication of a hydrogel SERS chip integrating a-AgNPs/CDs (carbon dots)((a–d) Steps including nanoparticle mixing, casting, freezing–thawing, and final chip formation; (c1,d1) Schematic of internal physical crosslinking network; (e) Optical image of completed hydrogel chip for real sample application [48]).

Figure 7

Hydrogel-integrated SERS platforms for detecting food toxins and illegal additives. (A) AgNPs@Agar hydrogel biosensor for melamine and penicillin G detection in camel milk powder. Silver nanoparticles are formed via in situ reduction with L-ascorbic acid in an agar matrix, creating a flexible SERS-active hydrogel membrane [198]; (B) Agarose hydrogel-SERS patch for formaldehyde (FA) detection using AuAg@SiO₂ nanoparticles. The hydrogel is functionalized with MBTH to form a derivative product upon reacting with FA, producing a strong Raman signal at 1273 cm⁻¹ under laser excitation [36]; (C) DNA hydrogel-based SERS platform for kanamycin detection using ligase-rolling circle amplification (L-RCA) ((i) L-RCA amplification forms a DNA hydrogel with magnetic responsiveness for SERS sensing; (ii) Synthesis of aptamer-functionalized gold nanoparticles (polyA-AuNPs) and preparation of gold core–shell SERS tags (GCNPs) with encapsulated Raman reporters; (iii) Hybridization of circularized padlock probes (CPP1 and CPP2) and ligation to form a double-targeted detection structure for kanamycin; (iv) Phi29 DNA polymerase-driven amplification and magnetic bead separation produce a DNA hydrogel embedded with GCNPs for enhanced SERS detection [238]). (D) PEGDA-AuNP nanocomposite hydrogel chip for detecting sulfur dioxide (SO₂) in wine ((a) Fabrication of the SERS chip using UV curing and punching to produce 7 mm discs; (b,c) UV–Vis spectra showing localized surface plasmon resonance of AuNP-loaded hydrogels; (d) SERS spectra under different SO₂ concentrations; (e) 3D Raman intensity plots confirming detection sensitivity [257]). (E) Molecularly imprinted hydrogel (MIH) for New Red azo dye detection. The hydrogel is synthesized with New Red as a template, forming specific binding cavities. After template removal, rebinding of the dye enables selective SERS signal generation with high recovery rates in beverage samples [259].

Figure 1

Overview of hydrogel-integrated SERS (Surface-Enhanced Raman Scattering) systems for food safety detection. (A) Sources, structure, and design: includes natural material surfaces, molecular structures, and schematic of a SERS-integrated hydrogel platform for analyte detection; (B) Structure–function relationship: (a) Synthesis of L-PNIPAm hydrogel via polymerization at 25 °C, followed by polydopamine (PDA) coating and sulfobetaine methacrylate (SBMA) functionalize to form LSAG hydrogel. (b) Thermoresponsive behavior of LSAG hydrogel: swelling at temperatures below the lower critical solution temperature (T < LCST) and shrinking at temperatures above (T > LCST); interconnected porous structure facilitates mass transfer; (C) Challenges and future perspectives: includes signal reproducibility, scalability of manufacturing, integration with 3D/4D printing, and development of stimuli-responsive degradation mechanisms; (D) Applications: shows the versatility of hydrogel-SERS platforms for detecting a wide range of food contaminants, including pathogens, pesticides, toxins, adulterants, and additives.