Real time label-free monitoring of plasmonic polymerase chain reaction products

The high mortality rate in developing countries stemming from poverty and diseases, and the pressure on healthcare budgets in developed countries have evoked a major concern in healthcare delivery. The need for less costly and patientcentered healthcare delivery brings point-of-care (POC) testing to the fore. POC devices help to eliminate the overhead associated with centralized bench-top laboratory instruments. Although handheld devices such as glucose biosensor strip exist, POC devices for molecular techniques such as Polymerase Chain Reaction (PCR) are new and emerging. PCR makes it possible to replicate DNA and generate millions of copies from a single strand. This finds applications in the medical field to identify and detect infectious diseases. Conventional PCR equipment is expensive and requires a significant amount of personnel time and space to setup and run in the laboratory. We have recently demonstrated a rapid and low-cost PCR thermocycler based on laser heating of gold nanoparticles suspended in the PCR tube. A critical aspect of PCR systems is the need to detect amplified products in real time. Here we show that by measuring the optical absorption of the suspended gold nanoparticles at a single wavelength during thermocycling, it is possible to detect amplified PCR products in real time. We investigate several different signal processing approaches in order to determine the most sensitive monitoring technique. This method makes it possible to distinguish between negative and positive PCRs with starting copy numbers as low as 10,000 genome copies per microliter.