Sensitive colorimetric detection of melamine in processed raw milk using asymmetrically PEGylated gold nanoparticles

In this study, the asymmetrically modified gold nanoparticles (AuNPs) with relatively large size are proposed for highly sensitive detection of melamine in raw milk. At first, we found that the sensitivity of colorimetric detection of melamine based on the conventional citrate-stabilized AuNPs could be significantly increased (limit of detection by visual observation: from 1 μmol L^-1 to 1 nmol L^-1) via enlarging the size of AuNPs (from 13 to 42 nm). However, the large-sized AuNPs are unstable in the solution, and the accuracy of relevant aggregation-based colorimetric detection method is dramatically decreased due to the rapid evanishment of the color of solution caused by the precipitation of large-scaled AuNPs aggregates after introducing melamine. In view of this limitation, the stabilizing agent polyethylene glycol (PEG) is introduced onto the selected area at the surface of AuNPs to stabilize the large-sized AuNPs sensor and further adjust the aggregation behavior in an oriented and controllable manner. As a result, the addition of melamine into asymmetrically PEGylated AuNPs with large size could still induce a color change from purple to blue, but the color change could be maintained in aqueous solution for a long time. This improvement is mainly caused by forming stable and oriented oligomers of AuNPs taking advantage of the asymmetrical modification of AuNPs with PEG chain in this system. Such a way of aggregation leads to remarkably enhanced long-term stability and an at least 2 orders of magnitude wider dynamic detection range (1.05 nmol L^-1 to 1 mmol L^-1) compared with conventional AuNPs sensors. Finally, such a novel asymmetrically modified AuNPs sensor is successfully applied to detect melamine in the raw milk with the satisfactory spiked recovery from 99.6% to 103.7% and RSD of about 3.0%. This work provides a sensitive and reliable method to detect melamine in pretreated milk, indicating a new avenue to make innovation on colorimetric sensors for real sample detection.