SERS characterization of dopamine and in situ dopamine polymerization on silver nanoparticles.

Dopamine (DA) regulates several functions in the central nervous system and its depletion is responsible for psychological disorders like Parkinson's disease. Several analytical approaches have been presented for DA detection in pathological diagnosis. SERS spectroscopy is a highly promising technique for the sensitive detection of DA. However, an improvement in its detection in aqueous solution is highly desirable for reliable quantification in biological fluids. In this work, we explored a label-free SERS approach for DA detection, employing two conventional methods to synthesize Ag colloids: reduction via citrates (c-AgNPs) and reduction via hydroxylamine (h-AgNPs), and SERS measurements were performed with a laser at 488 nm wavelength. Under these conditions, DA was identified through reproducible SERS spectra in the c-AgNP medium; however, the SERS spectra of DA in h-AgNP solution showed a completely different SERS profile. SERS band analysis revealed that DA in h-AgNPs was oxidized and converted into polydopamine (PDA), which was triggered after exposure to laser radiation. DA oxidation and PDA formation were followed over time through the SERS band profile at pH 7, 9 and 12. We found that in situ PDA formation started after 50 min of laser irradiation of DA at pH 7, while DA was quickly oxidized at pH 9 and 12. Here, we present a detailed SERS band analysis of PDA, which sheds light on the molecular steps in the pathway formation of the PDA structure. Spectroscopic analysis and characterization revealed that a long laser exposure time led to the formation of stable PDA complexes with AgNPs, which allowed us to propose a novel approach for synthesis of AgNP-PDA composites. In conclusion, to detect DA through a label-free SERS approach, c-AgNPs must be employed, while stable AgNP-PDA materials can be achieved with h-AgNPs and 488 nm laser excitation.

Distinction of Ecuadorian varieties of fermented cocoa beans using Raman spectroscopy.

Cocoa (Theobroma cacao) is a crop of economic importance. In Ecuador, there are two predominant cocoa varieties: National and CCN-51. The National variety is the most demanded, since its cocoa beans are used to produce the finest chocolates. Raman measurements of fermented, dried and unpeeled cocoa beans were performed using a handheld spectrometer. Samples of the National and CCN-51 varieties were collected from different provinces and studied in this work. For each sample, 25 cocoa beans were considered and each bean was measured at 4 different spots. The most important Raman features of the spectra were assigned and discussed. The spectroscopic data were processed using chemometrics, resulting in a distinction of varieties with 91.8% of total accuracy. Differences in the average Raman spectra of cocoa beans from different sites but within the same variety can be attributed to environmental factors affecting the cocoa beans during the fermentation and drying processes.