Enhanced photoluminescence of porous silicon nanoparticles coated by bioresorbable polymers

Abstract

A significant enhancement of the photoluminescence (PL) efficiency is observed for aqueous suspensions of porous silicon nanoparticles (PSiNPs) coated by bioresorbable polymers, i.e., polylactic-co-glycolic acid (PLGA) and polyvinyl alcohol (PVA). PSiNPs with average size about 100 nm prepared by mechanical grinding of electrochemically etched porous silicon were dispersed in water to prepare the stable suspension. The inner hydrophobic PLGA layer prevents the PSiNPs from the dissolution in water, while the outer PVA layer makes the PSiNPs hydrophilic. The PL quantum yield of PLGA/PVA-coated PSiNPs was found to increase by three times for 2 weeks of the storage in water. The observed effect is explained by taking into account both suppression of the dissolution of PSiNPs in water and a process of the passivation of nonradiative defects in PSiNPs. The obtained results are interesting in view of the potential applications of PSiNPs in bioimaging.

Figure 1

SEM images of uncoated nanoparticles after ultrasonic treatment. Inset shows size (diameter) distribution of PSiNPs deposited on c-Si substrate.

Figure 2

SEM images of coated nanoparticles after ultrasonic treatment. Inset shows size (diameter) distribution of PSiNPs.

Figure 3

FTIR absorption spectra of the samples: uncoated sample before storage in water (solid line), uncoated sample after storage in water (dashed line) and PLGA/PVA-coated sample after storage in water (dotted line). The storage was done in darkness for 2 weeks at room temperature.

Figure 4

Examples of PL spectra. PL spectra of uncoated (blue line) and coated (red line) samples before (solid lines) and after (dashed lines) 4 days storage. Inset shows photograph of aqueous suspensions of coated and uncoated PSiNPs under UV irradiation just after the preparation.

Figure 5

Time dependences of the integrated PL intensity of the samples: uncoated PSiNPs (blue triangles) and coated PSiNPs (red squares). Solid lines are results of nonlinear fitting by Equations 1 and 2, respectively.

Figure 6

Sketch of the samples. (a) A schematic view of the porous Si nanoparticle (PSiNP) coating process where brown color corresponds to silicon, violet to PLGA, green to PVA and light blue to water. Note that every PSiNP includes several smaller nanocrystals (nc-Si). (b) Magnification of the uncoated PSiNP surface contiguous to liquid water. Hydrophobic surface mostly consisted of Si-H bonds was colored black and hydrophilic one mostly consisted of Si-O bonds was colored yellow. (c) Magnification of the coated PSiNP surface. Violet and green lines correspond to PLGA and PVA molecules. Note that liquid water is not in direct contact with the silicon surface of PSiNPs due to the hydrophobic properties of PLGA.

Figure 7

Transients of the PL relaxation. Transients of coated PSiNPs are depicted by red squares and transients of uncoated PSiNPs are depicted by blue triangles (detection wavelength is 600 nm). Lines are results of fitting by Equation 5.