Near-infrared photoluminescence enhancement of N-acetyl-L-cysteine (NAC)-protected gold nanoparticles via fluorescence resonance energy transfer from NAC-stabilized CdTe quantum dots

Water-soluble N-acetyl-L-cysteine-protected gold nanoparticles (NAC-AuNPs) and NAC-stabilized cadmium telluride quantum dots (NAC-CdTeQDs) have been synthesized. The near-infrared (NIR) photoluminescence (PL) of NAC-AuNPs in various aqueous pH phosphate buffers has been studied in the presence of NAC-CdTeQDs with different core sizes. The NIR PL intensities of 1.7 nm and 2.4 nm sized NAC-AuNPs increases with NAC-CdTeQDs increase. The PL enhancement for 2.4 nm NAC-AuNPs is larger than 1.7 nm NAC-AuNPs and also increases with the core size of NAC-CdTeQDs from 1.9 to 3.2 and then to 3.7 nm. In addition, the PL enhancement effect initially increases with pH and then drops with a further increase in pH, attributed to the coulombic repulsion between the same negatively charged NAC-AuNP and NAC-CdTeQD molecules at pH >= 7. At pH 4.0, NAC-CdTeQDs can show quenching on NAC-AuNPs. In contrast, the PL of NAC-CdTeQDs in the visible region is strongly quenched by the presence of NAC-AuNPs. The quenching efficiency decreases with the core size of NAC-AuNPs from 1.7 to 2.4 and then to 2.9 nm attributed to the decrease in the collision frequency of larger core NAC-AuNPs with NAC-CdTeQDs. Furthermore, the quenching effect increases with the decrease of pH as the coulombic repulsion between NAC-CdTeQDs and NAC-AuNPs is minimized under a low pH environment such that the intermolecular distance between the interacted NPs is shortened. The quenching of yellow and red is smaller than green NAC-CdTeQDs by NAC-AuNPs at pH 6.0 but larger than green ones at pH 7.0. The results demonstrate that the interaction between NAC-AuNPs and NAC-CdTeQD is mainly governed by the fluorescence resonance energy transfer model and the collision frequency of the AuNPs and QDs.