A Fluorescence Resonance Energy Transfer Probe Based on DNA-Modified Upconversion and Gold Nanoparticles for Detection of Lead Ions

Abstract

We report a new sensor for the specific detection of lead ions (Pb²⁺) in contaminated water based on fluorescence resonance energy transfer (FRET) between upconversion nanoparticles (UCNPs) as donors and gold nanoparticles (Au NPs) as receptors. The UCNPs modified with Pb²⁺ aptamers could bind to Au NPs, which were functionalized with complementary DNA through hybridization. The green fluorescence of UCNPs was quenched to a maximum rate of 80% due to the close proximity between the energy donor and the acceptor. In the presence of Pb²⁺, the FRET process was broken because Pb²⁺ induced the formation of G-quadruplexes from aptamers, resulting in unwound DNA duplexes and separated acceptors from donors. The fluorescence of UCNPs was restored, and the relative intensity had a significant linear correlation with Pb²⁺ concentration from 0 to 50 nM. The sensor had a detection limit as low as 4.1 nM in a buffer solution. More importantly, the sensor exhibited specific detection of Pb²⁺ in complex metal ions, demonstrating high selectivity in practical application. The developed FRET prober may open up a new insight into the specific detection of environmental pollution.

Keywords: DNA; fluorescence resonance energy transfer; gold nanoparticles; lead ions; upconversion nanoparticles.

Figure 1

The schematic illustration of sensing assay constructed by UCNPs and Au NPs for Pb²⁺ detection.

Figure 2

TEM images of (A) NaYF₄: 18% Yb and 2% Er, (B) NaYF₄: 18% Yb and 2% Er @NaYF₄, (C) ligand-free UCNPs, and (D) DNA1-modified UCNPs (UCNPs-DNA1). (E) The upconversion fluorescence spectra of UCNPs with OA surface ligand (UCNPs-OA) and DNA aptamers (UCNPs-DNA1) under excitation of 980 nm. (F) The UV–vis absorption spectra of UCNPs-DNA1.

Figure 3

TEM images of (A) Au NPs and (B) DNA2-modified Au NPs (Au NPs-DNA2). (C) The UV–vis absorption spectra of Au NPs and Au NPs-DNA2. (D) The upconversion fluorescence spectra of UCNPs-DNA1 and the UV–vis absorption spectra of Au NPs-DNA2.

Figure 4

(A) The upconversion fluorescence spectra and (B) quenching efficiency of UCNPs-DNA1 after incubation at various concentrations of AuNPs-DNA2 in FRET system. (C) TEM image of FRET array fabricated from UCNPs-DNA1 and AuNPs-DNA2. (D) The fluorescence quenching efficiency with increasing incubation time.

Figure 5

(A) The restoration of fluorescence after incubation at different concentrations of Pb²⁺ in FRET system. (B) TEM image of FRET array fabricated from UCNPs-DNA1 and AuNPs-DNA2 after Pb²⁺ incorporation. (C) The relationship between relative fluorescence intensity (F–F₀)/F₀ and Pb²⁺ concentration. (D) Linear correlation between relative fluorescence intensity (F–F₀)/F₀ and Pb²⁺ concentration in the range of 0–50 nM.

Figure 6

(A) The fluorescence intensity of FRET array after incubation at different concentrations of Pb²⁺. (B) The fluorescence intensity of detection sensor after the incorporation of Pb²⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺, Hg²⁺, Cu²⁺, Zn²⁺, and Fe³⁺.