Ultrasensitive analyte detection with plasmonic paper dipsticks and swabs integrated with branched nanoantennas†
Paper-based substrates integrated with plasmonic nanostructures and combined with surface enhanced Raman spectroscopy (SERS) offer a flexible and lightweight platform for the ultrasensitive optical detection of analytes on any surface. Here, we incorporated multibranched gold nanoantennas (MGNs) on inexpensive filter paper to design MGN-paper dipsticks and swabs for SERS mediated sensing of chemicals, proteins, and pesticides adsorbed on fruits. MGNs are anisotropic nanostructures consisting of a core which serves as the antenna and protrusions that serve as emitters redistributing incident light. The nanoantenna effect gives rise to intense electromagnetic fields on the tips of the protrusions that enabled a detection of 100 pM of 1,4-benzenedithiol and 100 fM of human serum albumin labeled with indocyanine green with the MGN-paper dipsticks. Further, MGN-paper swabs enabled the detection of 62.5 pg of solid state 4-aminothiophenol on a planar surface, and 26.3 μg of methyl parathion adsorbed on an apple. Finite difference time domain simulations demonstrated that the nanoantenna effect can be systematically modulated by altering the core-to-protrusion ratio to generate a ∼65× enhancement in the electromagnetic fields localized on the protrusions which may ultimately result in sub-femtomolar to zeptomolar detection sensitivities.