Nanoparticle-Enhanced LIBS Mapping Uncovers Coffee-Ring Effect in Serum Analysis
Researchers from the Harbin Institute of Technology and Harbin Medical University Cancer Hospital have demonstrated how optimizing silver nanoparticle concentrations can mitigate the coffee-ring effect in nanoparticle-enhanced laser-induced breakdown spectroscopy (NELIBS), improving the accuracy of elemental analysis in bioliquid samples.
A common issue that researchers face when using nanoparticle-enhanced laser-induced breakdown spectroscopy (NELIBS) measurements is the coffee-ring effect (CRE), which occurs when a liquid droplet containing suspended particles dries, leading to a characteristic ring-like stain because of the outward flow of solvent. The coffee ring effect can negatively impact spectral accuracy.
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A recent study published in Spectrochimica Acta Part B: Atomic Spectroscopy conducted by researchers from the Harbin Institute of Technology and Harbin Medical University Cancer Hospital, investigated how the CRE affects spectral accuracy when analyzing bioliquid samples (1). The study also explores several strategies to mitigate the impact of the CRE.
Laser-induced breakdown spectroscopy (LIBS) helps determine the elemental composition of minerals (2). LIBS functions by producing a laser pulse to ablate the sample surface and forma plasma (2). The plasma is then used to excite the sample, emitting light, which is transferred through fiber optics and enters the spectrometer (2). Researchers then use the spectrometer to produce the spectral data and analyze it, which helps determine the concentration of each element (2).
However, obstacles exist with using NELIBS. Researchers often encounter a CRE, which can cause uneven elemental distribution, ultimately affecting measurement accuracy (1).
In their study, the research team evaluated CRE in bioliquid samples by investigating serum mixed with silver nanoparticles (Ag NPs) using LIBS mapping (1). The team employed the dry droplet method (DDM), where the serum-Ag NP mixture was dripped onto untreated silicon (Si) substrates and allowed to dry, forming a solid-state sample for analysis (1). By performing a two-dimensional (2D) grid scan with a 1064-nm pulsed Nd:YAG laser, the team generated false-color distribution images of various emitting species.
From these images, the researchers were able to see variations in the CRE patterns. The team observed that the intensity distributions of carbon (C), cyanide (CN), calcium (Ca), and magnesium (Mg) emissions formed clear boundaries between the center and the ring regions of the dried stain (1). In contrast, potassium (K) emission was predominantly concentrated in the center region (1).
The researchers also discovered a correlation between Ag NPs and the spectral intensity in the center region. The proportion of spectral intensity from the center region relative to the entire drop stain exceeded 60% and increased up to 98% with higher Ag NP concentrations (1). This result suggests that by optimizing the ratio of serum to Ag NPs, researchers can significantly reduce CRE-related inconsistencies in LIBS measurements (1).
The researchers also explored the enhancement effects of Ag NPs on spectral emissions. They found that when the volume ratio of serum to Ag NPs was adjusted to 1:2, the enhancement factors for the K I 766.49 nm line and Ca II 393.36 nm line reached their highest values of 2.27 and 1.90, respectively (1). As a result, they validated the widely accepted idea that Ag NPs play a crucial role in amplifying LIBS signal intensities (1). Because Ag NPs help amplify LIBS signal intensities, they can be used to improve the sensitivity and reliability of elemental analysis in biological samples (1).
Although previous investigations have acknowledged the CRE in dried bioliquid samples, few have explored its detailed impact on LIBS measurements. By employing LIBS mapping techniques, this study offers a comprehensive evaluation of CRE in serum-Ag NP drop stains and presents an effective strategy to mitigate its influence (1).
Based on the findings of the study, the researchers unveiled new insights into the CRE in NELIBS analysis. Future studies will be able to use this information to test and develop more reliable and precise elemental analysis in bioliquid samples, advancing the potential of LIBS in clinical applications (1).
References
- Zhang, X.; Li, X.; Chen, X.; Shi, M.; Ren, T. Evaluation of Coffee-ring Effect of Serum and Ag NPs Mixture Drop Stains Using Laser-induced Breakdown Spectroscopy Mapping. Spectrochimica Acta Part B: At. Spectrosc. 2025, 227, 107158. DOI: 10.1016/j.sab.2025.107158
- Thermo Fisher Scientific, How Does LIBS Work? Thermo Fisher Scientific. Available at: https://www.thermofisher.com/us/en/home/industrial/spectroscopy-elemental-isotope-analysis/portable-analysis-material-id/industrial-elemental-radiation-solutions/how-does-libs-work.html (accessed 2025-02-25).
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