An enzyme-responsive metal-enhanced near-infrared fluorescence sensor based on functionalized gold nanoparticles

Abstract

Near-infrared (NIR) fluorescence imaging is promising due to the high penetration depths and minimal levels of autofluorescence in living systems. However, it suffers from low fluorescent quantum yield, and metal-enhanced fluorescence (MEF) is considered to be a promising technique to overcome this. Stimuli-responsive NIR fluorescence enhancement shows remarkable potential for applications in medical imaging and diagnosis. Herein, we successfully fabricated an enzyme-responsive near-infrared sensor based on MEF by functionalizing gold nanoparticles with NIR fluorophores and enzyme-responsive self-aggregation moieties. The NIR fluorescence of fluorophores on the gold nanoparticles was significantly enhanced due to increases both in the light scattering intensity and in the radiative decay rate (k_r) of the NIR fluorophores, along with relatively small variation in the nonradiative decay rate. This novel strategy for NIR fluorescent sensors should be particularly promising for NIR fluorescence imaging of enzyme activities and early diagnosis based on rationally designed nanomaterials.

Scheme 1. Schematic illustration of the proposed mechanism of the enzyme-responsive fluorescence enhancement by NGal-NIR-AuNPs/CHO-NIR-AuNPs.

Fig. 1. (a) Time-dependent absorbance spectral changes of NGal-NIR-AuNPs/CHO-NIR-AuNPs (2.0 nM) in the presence of β-gal (1.0 μM). (b) Time-dependent fluorescence spectral changes (λ _ex = 680 nm) of NGal-NIR-AuNPs/CHO-NIR-AuNPs (1.0 nM) in the presence of β-gal (1.0 μM). (c) Relative fluorescence intensity in the absence or presence of β-gal, where the enhancement factor of Lip-Cy5.5m was normalized to 1. All of the samples were suspended in PBS (pH 7.4) at 37 °C.

Fig. 2. Morphological variation of functionalized gold nanoparticles, induced by enzymatic reaction in PBS at 37 °C. (a) TEM images of NGal-NIR-AuNPs/CHO-NIR-AuNPs (2.0 nM) in the absence (left) and presence (right) of β-gal (1.0 μM) after 5 h. Scale bars: 100 nm. (b) Time-dependent variation of the hydrodynamic diameter distributions of NGal-NIR-AuNPs/CHO-NIR-AuNPs (2.0 nM) measured by DLS after the addition of β-gal (1.0 μM).

Fig. 3. Light scattering spectral shift of NGal-AuNPs/CHO-AuNPs (1.0 nM) in the absence and presence of enzyme (6 h) in PBS (pH 7.4) at 37 °C.

Fig. 4. Plausible mechanism of the β-gal-responsive metal-enhanced NIR fluorescence enhancement of NIR-AuNPs by the consideration of the photophysical parameters.