Highly Bright Gold Nanowires Arrays for Sensitive Detection of Urea and Urease

Abstract

In this work, highly fluorescent gold nanowire arrays (Au NWs) are successfully synthesized by assembling Zn²⁺ ions and non-emissive oligomeric gold-thiolate clusters using mercaptopropionic acid both as a reducing agent and a growth ligand. The synthesized Au NWs exhibited strong bluish green fluorescence with an absolute quantum yield up to 32% and possessed ultrasensitive pH stimuli-responsive performance in the range of 7.0-7.8. Based on the excellent properties of the as-prepared nanowire arrays, we developed a facile, sensitive, and selective fluorescent method for quantitative detection of urea and urease. The fabricated nanoprobe showed superior biosensing response characteristics with good linearities in the range of 0-100 μM for urea concentration and 0-12 U/L for urease activity. In addition, this fluorescent probe afforded relatively high sensitivity with the detection limit as low as 2.1 μM and 0.13 U/L for urea and urease, respectively. Urea in human urine and urease in human serum were detected with satisfied results, exhibiting a promising potential for biomedical application.

Keywords: assembly induced emission; enhanced fluorescence; gold nanowire arrays; self-assembly; urea and urease.

Figure 1

The effect of pH (A) and Zn-to-Au ratio (B) on the fluorescence intensity of Au NWs. (C) The change of fluorescence intensity at different temperature during synthesis of Au NWs.

Figure 2

Characterization of the synthesized Au NWs. TEM (A) and SEM (B) image of Au NWs. FT-IR (C) of MPA and Au NWs. DLS (D) of Au NWs after introducing Zn²⁺ ions 0–1.5 h.

Figure 3

(A) UV-vis absorption spectra of Au₄ NCs (black) and Au NWs (red). (B) Fluorescence spectra at different excitation wavelengths and excitation spectrum of Au NWs. (C) Photographs taken under visible light and UV light of Au₄ NCs, Au NWs in water and Au NWs powder. (D) The plot of the fluorescence intensity of Au NWs (excitation at 360 nm) recorded at 485 nm versus the pH value.

Scheme 1

Fluorescence signal-off nanoprobe based on Au NWs for urea and urease detection.

Figure 4

(A) The fluorescence spectra of Au NWs, Au NWs + urea, Au NWs + urease, and Au NWs + urea + urease. (B) The effect of the concentration of urea (square) and ammonia (circle) on the fluorescence intensity of Au NWs. (C) DLS with different concentrations of ammonia, (D) fluorescence lifetime, and (E) UV-vis absorption of Au NWs with different amount of ammonia.

Figure 5

(A) Fluorescence emission spectra of Au NWs sensing system upon adding various concentrations of urea at 12 U/L urease (from top to bottom: 0, 5, 10, 25, 50, 75, 100, 125, 150, 175, and 200 μM, respectively), The inset of (A) showed the corresponding photographs and relationship between the fluorescence intensity ratio F/F₀ and varied levels of urea. (B) Linear correlation between F/F₀ and urea concentrations. (C) Fluorescence emission spectra of Au NWs sensing system upon adding various concentrations of urease at 500 μM urea (from top to bottom: 0, 0.2, 1, 2, 4, 6, 8, 10, 12, 15 and 20 U/L, respectively). The inset of (C) showed the corresponding photographs and relationship between the fluorescence intensity ratio F/F₀ and varied levels of urease. (D) Linear correlation between F/F₀ and urease activities.

Figure 6

The selectivity of the established sensing system to coexisting interference substances. 0.5 mM urea, 8 U/L urease, 10 μM metal ions, 100 μM amino acids, and 100 mg/L proteins.