Evaluation of coffee-ring effect of serum and Ag NPs mixture drop stains using laser-induced breakdown spectroscopy mapping

Abstract

Nanoparticle-enhanced laser-induced breakdown spectroscopy (NELIBS) has shown great potential in improving the sensitivity of normal LIBS. However, the coffee-ring effect (CRE), often encountered when using the dry droplet method (DDM), may affect the accuracy of NELIBS measurements on bioliquid samples. In this paper, evaluation of the CRE of serum and silver nanoparticle (Ag NPs) mixture drop stains was investigated using LIBS mapping. Serum-Ag NPs mixture was dripped onto normal untreated silicon (Si) substrates for solid-liquid conversion using DDM. A two-dimensional grid scan of the whole drop stain was performed using a 1064 nm pulsed Nd: YAG laser, to generate the false-color distribution images of emitting species. The intensity distribution of the C I 247.86 nm line was used to distinguish the serum drop from the Si substrate, and to distinguish the center and ring regions of the stain. The CRE patterns varied for different emitting species. The distribution images of C, CN, Ca, and Mg emissions showed clear boundaries between the center and the ring regions. However, the K emission concentrated in the center region. The morphological variations of the coffee-rings were related to the mixing volume ratio of serum and Ag NPs. With the increase of Ag NPs, the distribution of emitting species in the center region became more uniform, and the CRE can be mitigated by adjusting the ratio of serum and Ag NPs. The intensities of the center and the ring regions were retrieved and evaluated. The percentage of the center region spectral intensity was more than 60 % relative to the whole drop stain, and gradually increased up to 98 % with the increase of Ag NPs. The variation trend of the spectral intensity of the center region was similar to that of the whole droplet. The enhancement effect of the Ag NPs on the spectral emissions was further investigated. When the volume ratio of serum to Ag NPs was 1:2, the highest enhancement factors for K I 766.49 nm line and Ca II 393.36 nm line were obtained as 2.27 and of 1.90, respectively. This work showed the potential of LIBS mapping in evaluation of the CRE of liquid drop stains. The results could provide important references for LIBS measurements of serum-Ag NPs drop stains on normal untreated solid substrates.