Quantitative SERS Detection of Uric Acid via Formation of Precise Plasmonic Nanojunctions within Aggregates of Gold Nanoparticles and Cucurbit[n]uril

Summary

A host-guest complex of cucurbit[7]uril and uric acid was formed in an aqueous solution before adding a small amount into Au NP solution for quantitative surface-enhanced Raman spectroscopy (SERS) sensing using a modular spectrometer.

Abstract

This work describes a rapid and highly sensitive method for the quantitative detection of an important biomarker, uric acid (UA), via surface-enhanced Raman spectroscopy (SERS) with a low detection limit of ~0.2 μM for multiple characteristic peaks in the fingerprint region, using a modular spectrometer. This biosensing scheme is mediated by the host-guest complexation between a macrocycle, cucurbit[7]uril (CB7), and UA, and the subsequent formation of precise plasmonic nanojunctions within the self-assembled Au NP: CB7 nanoaggregates. A facile Au NP synthesis of desirable sizes for SERS substrates has also been performed based on the classical citrate-reduction approach with an option to be facilitated using a lab-built automated synthesizer. This protocol can be readily extended to multiplexed detection of biomarkers in body fluids for clinical applications.

Introduction

Uric acid, which is the end product of metabolism of purine nucleotides, is an important biomarker in blood serum and urine for the diagnosis of diseases such as gout, preeclampsia, renal diseases, hypertension, cardiovascular diseases and diabetes^1,2,3,4,5. Current methods for uric acid detection include colorimetric enzymatic assays, high performance liquid chromatography and capillary electrophoresis, which are time-consuming, expensive and require sophisticated sample preparation⁶

Protocol

1. Synthesis of Au NPs

1. Synthesis of Au seeds via the conventional Turkevich method²⁶
   1. Prepare 10 mL of 25 mM HAuCl₄ solution by dissolving 98.5 mg of HAuCl₄· 3H₂O precursor with 10 mL of deionized water in a glass vial.
      NOTE: Transfer a small amount of HAuCl₄ precursor into a weighing boat and use a plastic spatula instead of metallic spatula to weigh out the crystals because HAuCl₄ precursor will corrode metal labware. The weighing step should be performed as swiftly as possible, since HAuCl₄ is hygroscopic and will therefore increase its weig....

Results

In the presented Au NP synthesis, the UV-Vis spectra show a shift of the LSPR peaks from 521 nm to 529 nm after 10 growing steps (Figure 4A,B) while the DLS data shows a narrow size distribution as the size of Au NPs increase from 25.9 nm to 42.8 nm (Figure 4C,D). The average sizes of G0, G5 and G10 measured from TEM images (Figure 4E) are 20.1 ± 2.1 nm, 32.5 ± 2.3 nm and 40.0 ± 2.2 nm respectively,.......

Discussion

The automated synthesis method described in the protocol allows Au NPs of increasing sizes to be reproducibly synthesized. Although there are some elements that still need to be carried out manually, such as the fast addition of sodium citrate during the seed synthesis and checking periodically to ensure that the PEEK tubing is secure, this method allows Au NPs of large sizes (up to 40 nm), which would usually require multiple manual injections of HAuCl₄ and sodium citrate, to be synthesized via continuous add.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
40 nm gold nanoparticles	NanoComposix	AUCN40-100M	NanoXact, 0.05 mg/ mL, bare (citrate)
Centrifuge tube	Corning Falcon	14-432-22	50 mL volume
Cucurbit[7]uril	Lab-made		see ref. 19
Gold(III) chloride trihydrate	Sigma aldrich	520918	≥99.9% trace metals basis
Luer lock disposable syringe	Cole-Parmer	WZ-07945-15	3 mL volume
Luer-to-MicroTight adapter	LuerTight	P-662	360 μm outer diameter Tubing to Luer Syringe
PEEK tubing	IDEX	1572	360 μm outer diameter, 150 μm inner diameter
PEEK tubing cutter	IDEX	WZ-02013-30	Capillary Polymer Chromatography Tubing Cutter For 360 µm to 1/32" OD tubing
Raman spectrometer	Ocean Optics	QE pro
Sodium citrate tribasic dihydrate	Sigma aldrich	S4641	ACS reagent, ≥99.0%
Sonicator
Standard Probe	Digi-Sense	WZ-08516-55	Type-K
Syringe pump	Aladdin	ALADDIN2-220	2 syringes, maximum syringe volume 60 mL
Thermocouple thermometer	Digi-Sense	WZ-20250-91	Single-Input Thermocouple Thermometer with NIST-Traceable Calibration
ThermoMixer	Eppendorf	5382000031	With an Eppendorf SmartBlock for 50 mL tubes
Uric acid	Sigma aldrich	U2625	≥99%, crystalline