A strategy to prevent signal losses, analyte decomposition, and fluctuating carbon contamination bands in surface-enhanced Raman spectroscopy

Signal losses and fluctuating carbon contamination bands are bottlenecks in the application of surface-enhanced Raman spectroscopy (SERS) for reliable chemical analysis. They originate mainly from prolonged laser irradiation of the sample during data collection, which causes analyte decomposition and/or loss of the enhancing capabilities of the adsorption site. In this work, a laser illumination/signal collection technique, the multiple points collection (MPC) method is introduced to circumvent these problems. The MPC method is based on the use of a pair of galvanic mirrors to scan the laser beam rapidly and steadily across the sample surface. Each position is irradiated for <10 mu s, at a rate of similar to 0.5 Hz. The SER spectrum is obtained by summing the signals collected from a large array of non-overlapping sample points. The MPC is compared with the conventional single point collection method, in which the laser beam is statically focused onto a particular spot and the scattered signals acquired. The MPC has the following advantages: (1) illumination and collection efficiencies are not compromised, (2) signal losses originating from analyte decomposition and/or alteration of the enhancing capabilities of the adsorption site are avoided, (3) high-quality SER spectra for analytes such as biomolecules and dipicolinic acid (a common marker for bacteria spores) can be easily obtained, and (4) the occurrence of broad amorphous carbon bands and the commonly observed temporal fluctuations in SERS are prevented. The success of the MPC is attributed to the reduction of local sample heating, as the time interval between the laser irradiations of a spot is much longer than the actual irradiation time itself.