Aqueous red-emitting silicon nanoparticles for cellular imaging: Consequences of protecting against surface passivation
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Katye Fichter and Tania Vu Department of Biomedical Engineering, Oregon Health & Sciences University, Portland, Oregon 97239
Micah Eastman and Jun Jiao Department of Physics, Portland State University, Portland, Oregon 97201
Andrea M. Gofortha) Department of Chemistry, Portland State University, Portland, Oregon 97201 (Received 27 July 2012; accepted 24 October 2012)
Stable, aqueous, red-to-near infrared emission is critical for the use of silicon nanoparticles (Si NPs) in biological fluorescence assays, but such Si NPs have been difficult to attain. We report a synthesis and surface modification strategy that protects Si NPs and preserves red photoluminescence (PL) in water for more than 6 mo. The Si NPs were synthesized via high temperature reaction, liberated from an oxide matrix, and functionalized via hydrosilylation to yield hydrophobic particles. The hydrophobic Si NPs were phase transferred to water using the surfactant cetyltrimethylammonium bromide (CTAB) with retention of red PL. CTAB apparently serves a double role in providing stable, aqueous, red-emitting Si NPs by (i) forming a hydrophobic barrier between the Si NPs and water and (ii) providing aqueous colloidal stability via the polar head group. We demonstrate preservation of the aqueous red emission of these Si NPs in biological media and examine the effects of pH on emission color. I. INTRODUCTION
Semiconductor nanoparticles (NPs) have received widespread attention in recent years for their size-tunable ultraviolet (UV)-to-near infrared (NIR) light emission governed by the quantum confinement effect. These photoluminescent particles, with size-tunable, narrow emission and broad absorption spectra, have been exploited in numerous applications, including in light-emitting/harvesting devices1,2 and as handles for biomedical tracking and imaging.3,4 Although predominantly, direct band gap semiconductor NPs, i.e., quantum dots (QDs), have thrived in semiconductor NP synthesis and application studies, attention has more recently turned toward the development of potentially less toxic, particularly relative to II–VI Cd- and Pb-based QDs, Si NP fluorophores after the observation of efficient visible photoluminescence (PL) from porous silicon containing nanometer-sized Si domains in the 1990s.5 Observation of efficient visible PL from nanometer-sized, indirect band gap Si has been attributed to relaxation of the momentumforbidden radiative exciton recombination across the indirect band gap with increased certainty in carrier position on the nanoscale.6,7 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.377 216
J. Mater. Res., Vol. 28, No. 2, Jan 28, 2013
http://journals.cambridge.org
Downloaded: 06 Oct 2014
Despite potential toxicity and compatibility advantages in using fluorescent Si NPs over CdQ and PbQ (Q 5 S, Se, Te) QDs in both devices (e.g., solely Si-based optoelectronics)8–10 and biomedical applications,4,11,12 synthetic control over Si NP emission color and quantum yield (QY) is poor relative to II–VI
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