Luminescence decay dynamics and trace biomaterials detection potential of surface-functionalized nanoparticles

We have studied the luminescence decay and trace biomaterials detection potential of two surface-functionalized nanoparticles, poly(ethylene glycol) bis(carboxymethyl) ether-coated LaF₃:Ce,Tb (∼20 nm) and thioglycolic acid-coated ZnS/Mn (∼5 nm). Upon UV excitation, these nanoparticles emitted fluorescence peaking at 540 and 597 nm, respectively, in solution. Fluorescence imaging revealed that these nanoparticles targeted the trace biomaterials from fingerprints that were deposited on various nonporous solid substrates. Highly ordered, microscopic sweat pores within the friction ridges of the fingerprints were labeled with good spatial resolutions by the nanoparticles on aluminum and polymethylpentene substrates, but not on glass or quartz. In solution, these nanoparticles exhibited multicomponent fluorescence decays of resolved lifetimes ranging from nano-to microseconds and of average lifetimes of ∼24 and 130 μs for the coated LaF₃:Ce,Tb and ZnS:Mn, respectively. The long microsecond-decay components are associated with the emitters at or near the nanocrystal core surface that are sensitive to the size, surface-functionalization, and solvent exposure of the nanoparticles. When the nanoparticles were bound to the surface of a solid substrate and in the dried state, a decrease in the microsecond decay lifetimes was observed, indicative of a change in the coating environment of the nanocrystal surface upon binding and solvent removal. The average decay lifetimes for the surface-bound ZnS :Mn in the dried state were ∼60, 30, and 11 μs on quartz, aluminum, and polymethylpentene, respectively. These values were still 2 orders of magnitude longer than the typical fluorescence decay background of most substrates (e.g., ∼0.36 μ for polymethylpentene) in trace forensic evidence detections. We conclude that coated ZnS: Mn nanoparticles hold great promise as a nontoxic labeling agent for ultrasensitive, time-gated, trace evidence detections in nanoforensic applications.