Label-free detection of amino acids using gold nanoparticles in electrokinetic chromatography–thermal lens microscopy

To achieve label-free detection of amino acids in capillary-based electrokinetic chromatography–thermal lens microscopy (EKC–TLM), gold nanoparticles (GNPs), which possess the absorption around 500 nm attributed to surface plasmon resonance (SPR), were added to the background solution (BGS). Since the SPR absorption of the GNPs exhibits a sensitive response toward environmental changes and the degree of aggregation, the sensitive detection of non-absorbing species is expected by using the GNPs in EKC–TLM (GNP–EKC–TLM). In the GNP–EKC–TLM analysis of glutamic acid (Glu), a sharp peak was observed when the GNPs were added to the BGS. The plot of the peak area of Glu against its concentration gave a good linear relationship and the limit of detection was estimated to be 25 μg/mL. Furthermore, a baseline separation of lysine and Glu was successfully achieved. Thus, the EKC separation and label-free TLM detection of the amino acids can be realized only by adding the GNPs into the BGSs.