Chain length dependence and sensing capabilities of the localized surface plasmon resonance of silver nanoparticles chemically modified with alkanethiol self-assembled monolayers

In this paper, we explore the optical properties of Ag nanoparticles chemic ally modified with alkanethiol self-assembled monolayers (SAMs) by measurin g the localized surface plasmon resonance (LSPR) spectrum using UV-vis exti nction spectroscopy. For all the experiments presented here, the Ag nanopar ticles were fabricated using the technique of nanosphere lithography (NSL) and had in-plane widths of 100 nm and out-of-plane heights of 50 nm. We fir st demonstrate that unmodified nanoparticles are extremely susceptible to s light changes in 3-dimensional structure when exposed to various solvents. These structural effects can have dramatic effects on the extinction maximu m, lambda (max), of the LSPR shifting it to the blue by over 100 nmr The si gnificant discovery reported here is that lambda (max) for NSL fabricated A g nanoparticles is extremely sensitive to the SAM properties. We will demon strate the following new features: (1) lambda (max) of the LSPR linearly sh ifts to. the red 3 nm for every carbon atom in the alkane chain; (2) spectr al shifts as large as 40 nm are caused by only 60 000 alkanethiol molecules per nanoparticle, which corresponds to only 100 zmol of adsorbate; and (3) the nanoparticles' sensitivity to bulk external environment is only attenu ated by 20% when the nanoparticles are modified with the longest chain alka nethiol (1-hexadecanethiol, similar to2 nm). Experimental extinction spectr a were modeled by using Mie theory for Ag nanospheres with dielectric shell s intended to mimic the self-assembled monolayer (SAM) in thickness and ref ractive index. We find that the Mie theory qualitatively predicts the exper imentally observed trend that lambda (max) linearly shifts to the red with respect to shell thickness, pr alkanethiol chain length; however, the theor y underestimates the sensitivity by approximately a factor of 4. Excellent correlation between theory and experiment was observed when Mie theory was, used to predict the degree of attenuation in LSPR sensitivity to bulk exter nal environment when the nanoparticle is encapsulated in a dielectric shell similar to an alkanethiol SAM. Finally, we demonstrate that Ag nanoparticl es modified with functionalized SAMs can be used in sensing applications. H ere, we show that the LSPR shifts to the red 5 nm with the adsorption of th e polypeptide poly-L-lysine (PL) to Ag nanoparticles modified with deproton ated carboxylate groups from 11-mercaptoundecanoic acid (11-MUA). Furthermo re, we will show that this system behaves reversibly and exhibits no detect able nonspecific binding.