A low-cost, monometallic, surface-enhanced Raman scattering-functionalized paper platform for spot-on bioassays

Abstract We demonstrate the fabrication and spot-on bioassay application of a low-cost, monometallic, surface-enhanced Raman scattering (SERS) paper platform using gold nanoparticle (AuNP)-loaded screen printing inks. The AuNPs were used to enhance the Raman intensity through a surface plasmon resonance (SPR)-driven optical property and sodium carboxymethylcellulose (CMC) was used as a viscous ink to create uniform distribution of the AuNPs on the paper substrate. To minimize the coffee ring effect and uniformly disperse the nanoparticles, the size of citrate-capped AuNPs, the CMC concentration, the volume ratio of CMC solution and AuNPs, and the printing cycles were optimized. Two printing cycles of optimized ink with a 7:1 mixture of 2-wt% CMC and AuNPs produced the strongest SERS effect and an enhancement factor of 1.8 × 104 with a SERS paper platform based on a Rhodamine B probe molecule. The platform exhibited high reproducibility, with less than 5% spot-to-spot variation in Raman intensity. Furthermore, the SERS spectra of normal and two virus-infected tear biofluids were comparable to the SERS spectral findings of gold-deposited SERS substrates. The enhanced SERS activity and high reproducibility of a low-cost, flexible, portable, and lightweight paper platform suggest the potential of point-of-care applications of biofluids in fields ranging from clinical analysis to industry.