Aptamer-functionalized AuNPs for the high-sensitivity colorimetric detection of melamine in milk samples

Abstract

Although aptamer-functionalized AuNPs technology exhibits great potential in analytical and biological chemistry, direct analysis of molecules at a low concentration using colorimetric assay remains challenging. The development of intuitive methods has attracted interest for promising detection of melamine in milk samples due to a demand for stable and understandable process. In this study, we propose a rapid and facile colorimetric measurement method of melamine combined aptamer-functionalized AuNPs in contaminated milk samples. To realize the high stability and the lower limit of detection, the aptamer-functionalized surface of AuNPs via a coordinated bond was used in combination with ultra-sonication. The kinetics of this colorimetric assay based on aptamer-functionalized AuNPs was analyzed to illustrate that the higher the concentration of melamine, the faster the aggregation of AuNPs induced. The sensitivity, selectivity, limit of detection and recovery rate were sufficiently validated to understand the measurement principle of melamine using aptamer-functionalized AuNPs. The calibration curve established by the absorption peak ratio (A640 /A520) was linear in the concentration range of 0~1μM of melamine in aqueous solutions with the correlation coefficient (R2) of 0.986 and the limit of detection (LOD) of 22 nM, whereas, the correlation coefficient (R2) of 0.998 and the LOD of 14.9 nM were achieved at the concentration of melamine below 0.5 μM in milk samples. The optimized performance of this colorimetric assay of melamine using aptamer-functionalized AuNPs in milk samples was obtained with 100 μL of 13 nm AuNPs solution, 40 μL of 1 μM (100 dilutions) aptamers and the pre-reaction time of 30 min. This simple colorimetric measurement of melamine using aptamer-functionalized AuNPs provides a promising target for various applications of the sample source with complex sample matrices.

Fig 1

TEM image of the AuNPs solution (a) and UV-Vis absorption spectra of AuNPs solution (b).

Fig 2. The schematic illustration of the colorimetric assay of melamine using aptamers-functionalized AuNPs.

Fig 3. The effect of NaCl concentrations indicated on the absorption spectrum.

Fig 4. Reaction time with different known concentrations of melamine added intoAuNPs solution.

Fig 5

UV-Vis absorption spectra of aptamer-AuNPs solution obtained from different concentrations of melamine samples in aqueous solutions (Fig 5A) and milk samples (Fig 5C), sensitivity of this colorimetric assay system for melamine detection in aqueous solutions (Fig 5B) and in milk solution (Fig 5D), respectively. The concentrations of melamine are 0, 0.1, 0.2, 0.5, 1.0 and 2.0 μM.

Fig 6. The selectivity of this colorimetric method obtained by adding10 μM of fructose, lactose, glucose and ammeline into the aqueous solution and milk samples, respectively.