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.
Highlights
Melamine is an organic compound that is commonly found in the form of nitrogen-rich white crystals, and is synthesized from urea decomposing into cyanic acid with an intermediate step
Some conventional methods have already been employed in laboratory have already been utilized to detect melamine, such as capillary electrophoresis[4], chemiluminescence [5], electrochemical immunosensors [6], high performance liquid chromatography/high performance liquid chromatography-mass spectrometry (HPLC/HPLC-MS)[7,8,9] and antibody-based immunological method [10]
Melamine in milk products has been detected on-site and real-time by using a reliable and highly sensitive colorimetric sensing method significantly involving the aggregation of AuNPs, the colorimetric assay of melamine using AuNPs in our recently published paper is found to be susceptible to complex solution conditions [45]
Summary
Melamine is an organic compound that is commonly found in the form of nitrogen-rich white crystals, and is synthesized from urea decomposing into cyanic acid with an intermediate step. It is imperative to explore a new, efficient and stable method to detect melamine in milk samples To achieve this goal, the surface of AuNPs needs to be functionalized with melamine aptamers via the coordinated bond, which allows aptamers to bind the melamine to trigger the aggregation of AuNPs in the presence of salt [46]. The kinetic analysis of this colorimetric assay of melamine using aptamer-functionalized AuNPs was validated This strategy involving the coordination chemistry is applicable for complex sample matrixes (such as food) since the presence of additional salts, DNA, proteins and small molecules would significantly affect aggregation of AuNPs
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