Surface-engineering of Fe(III)-polydopamine nanoparticles enabling high-resolution optical-electrochemical dual-mode imaging of various latent fingerprints.

High-resolution visualization of latent fingerprints (LFPs) is crucial for effective crime scene investigation. The small particle reagent (SPR) method, known for its operational simplicity, broad applicability, and reliable development performance, shows great potential in LFPs development. However, studies that systematically investigate the interaction mechanisms between materials and LFPs while enabling high-resolution development remain scarce. In this study, we performed systematic surface-engineering modification of Fe(III)-polydopamine (PDA) nanoparticles, achieving high-resolution optical-electrochemical dual-mode imaging of LFPs. The surface charge and wettability of the materials were regulated by adding triethanolamine (TEA), 3-aminopropyltriethoxysilane (APTES), and cetyltrimethylammonium chloride (CTAC) to Fe(III)-PDA aqueous suspension, producing five nanoparticles with all combinations of positive/negative charges and hydrophilicity/hydrophobicity. It was found that positively charged and hydrophobic nanoparticles were more suitable as developing agents for LFPs due to strong interfacial adhesion between them. This approach enabled the visualization of level 1 to level 3 features of LFPs on polyvinylidene fluoride (PVDF) membranes, as well as level 1 and level 2 features of LFPs transferred via PVDF membranes from various substrates. Subsequent experiments involving consecutive fingerprint deposition and aged fingerprint development further demonstrated the high sensitivity and stability of this strategy. Notably, leveraging the strong electrochemical activity of Fe(III), which induces differential electrochemical reactivity between the ridge and furrow regions of fingerprints, we integrated PVDF membrane transfer with scanning electrochemical microscopy (SECM) to effectively reduce interference from complex substrate backgrounds and achieve high-resolution imaging of LFPs. These findings indicate that our developed "Fe(III)-PDA-TEA-CTAC + PVDF membrane" strategy opens a new avenue for the efficient development and selection of reagents toward LFP imaging.