Plasmonic Gradient Arrays for Rapid Screening of Surface-Enhanced Raman Scattering Efficiency: Particle Libraries of Gold Nanostars

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ABSTRACT: The efficiency of signal enhancement for surface-enhanced Raman
scattering (SERS) spectroscopy is a crucial parameter for the design and  development of nanoparticle-based sensing applications. However, screening SERS capabilities of anisotropic nanoparticles by iterative synthesis is time consuming, and their prediction still suffers from the complex nonlinear relationships of morphological and electromagnetic properties. We present an approach to use a macroscopic gradient array of substrate-supported nanoparticles for rapid screening of their SERS efficiencies. The gradient represents a “plasmonic library” of colloids synthesized by two-step post-modification of a monolayer of randomly close-packed gold nanospheres covered with poly(N-isopropylacrylamide) shells. A first chemical overgrowth process yields a continuous gradient of seed particles, with diameters ranging between 10 and 60 nm.

Subsequently, the seeds are further grown into nanostars (NSs) with spiky tips, which improve their SERS-enhancing capabilities.

Raman mapping along the gradient provides rapid and reliable quantification of the specific SERS efficiencies for the whole library, as well as correlation of their optical and structural properties. By ensuring that the number and density of particles in the Raman excitation volume remain constant, the most appropriate synthetic conditions for efficient SERS can be readily identified on a single screening. As a proof of application, we screened the SERS performance of a library of NSs and applied the selected best candidates for the detection of the bacterial biomarker pyocyanin. This bacterial quorum sensing signaling molecule was quantitatively detected within a linear dynamic range between 10−7 and 10−5 M, suitable for clinical applications.

Growth of Substrate-Supported Nanospheres.

For the overgrowth of the substrate-supported seed particles, the substrates were dipped into a growth solution and retracted at 1 mm min−1 using a dip-coater (NadeTech ND-R) at 25 °C maintained by a water bath. The growth solution was prepared by slowly adding 417 μL of HAuCl4 (0.1 M) to 100 mL of CTAB (0.1 M), followed by dropwise addition of 588 μL of a fresh AA solution (0.1 M), both under
vigorous stirring. The substrates were then washed by immersing them twice in water for 1 h and finally dried by N2 flow. To improve the adhesion of the gradient seed array, substrates were cured for 15 min on an Al foil-covered heating plate set to 100 °C and allowed to cool down.