Photoluminescent Silicon Nanocrystals with Mixed Surface Functionalization for Biophotonics
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Photoluminescent Silicon Nanocrystals with Mixed Surface Functionalization for Biophotonics Folarin Erogbogbo and Mark T. Swihart Chemical and Biological Engineering, University at Buffalo (SUNY), 303 Furnas Hall, Buffalo, NY, 14260-4200 ABSTRACT Orange-emitting photoluminescent silicon nanocrystals have been grafted with pairs of compounds to produce mixed monolayers on their surfaces. Vinyl acetic acid (VA) is used to enhance dispersibility in water by providing carboxyl termination, while alkyl groups are used to improve photoluminescence (PL) stability. The grafting of these molecules to the surface is enabled by an etching procedure that leaves a hydrogen-terminated surface on the silicon nanocrystals. Multiple molecules can be grafted in series by taking advantage of the residual hydrogen present after initial grafting with a first compound. Multiple molecules can also be grafted in parallel by allowing them to compete for surface reactive sites and varying their concentrations in solution to control the composition of the mixed surface layer. Both series and parallel grafting result in particles that have a mixture of two distinct molecules on their surface, and thereby allow control of the particle dispersibility in different solvents and the density of reactive groups for subsequent functionalization steps. Optimizing the surface composition may allow production of particles that can be reacted with proteins via carbodiimide linking chemistries while maintaining their PL and their dispersibility in water and buffer solutions. Here we focus on the parallel grafting approach. INTRODUCTION Semiconductor nanocrystals, or quantum dots (QDs), posses unique intrinsic optical properties that make them valuable as photoluminescent probes for biological sensing and imaging[1]. Although silicon QDs may have advantages in these applications, such as low cost and low toxicity, they have thus far not been widely used. Making nanoparticles dispersible in water is an important step towards making them more useful in the fields of biological, optical, and electronic engineering[2]. Most reported Si QDs are dispersible in nonpolar organic solvents because efficient grafting techniques exist to functionalize silicon nanoparticles with organic compounds[3-5]. These techniques are used to obtain organic coated Si QDs with photoluminescence spanning the visible spectrum[3,4,6]. Few procedures exist to make silicon QDs dispersible in water. This has been achieved by grafting them with acrylic acid [7,8], allylamine[9] or an organic compound that undergoes further derivatization resulting in the attachment of a biomolecule. Currently, Si nanocrystals that are dispersible in polar solvents have PL that is either unstable or restricted to a narrow range of the visible spectrum. Thus, new approaches remain of interest. Tilley et al.[9] obtained water dispersible particles with blue luminescence by reacting silicon particles with allylamine. Li and Ruckenstein reported red emitting water soluble particles grafted with pol
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