In this study, gold nanoparticles (AuNPs) were synthesized using the Turkevich method. This article explains the didactic step-by-step synthesis, showing pictures of the entire process, including a well-explained mechanism and characterization study. Synthesis involves the reduction of NaAuCl₄ using sodium citrate at high temperatures (approximately 90 °C). The two main mechanisms used to explain AuNPs synthesis via the Turkevich method are also discussed. The first mechanism considers that a nanowire intermediary and the other proposes that aggregate intermediates are not formed at any time during the synthesis. The materials (NaAuCl₄ and AuNPs) were characterized using UV-Vis spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and dynamic light scattering (DLS). The UV-Vis spectrum exhibits an absorption maximum at 521 nm because of the surface plasmon resonance (SPR) absorption band of the AuNPs. The SEM images of NaAuCl₄ show crystals with cubic shapes, while the AuNPs have an average particle size of approximately 16–25 nm and particles that appear mainly spherical. To confirm the particle shapes, AFM was conducted, and it was possible to clearly observe individual spherical nanoparticles and their aggregates, and the average diameter of these AuNPs was approximately 12–19 nm. The XRD pattern of AuNPs showed four main characteristic peaks corresponding to the (111), (200), (220), and (311) planes, confirming the presence of cubic (FCC) gold. The DLS presented an average particle size of 3.3 ± 0.9 nm and a polydispersity index (PDI) of 0.574. AuNPs were synthesized using a simple and rapid method. The resulting spherical and ultra-small particles can be used in several applications.

A disposable voltammetric immunosensor was developed to measure breast cancer biomarker 15-3 (CA 15-3) in human saliva and serum samples. Screen-printed paper-based electrodes (f-SPE) previously fabricated by our research group using homemade conductive inks were used as transducers, which were later modified only with gold nanoparticles to immobilize anti-CA 15-3 antibodies. The sensor was operated using antigen–antibody interactions in conjunction with a redox species (ferrocyanide potassium) for the indirect determination of the CA 15-3 antigen. The device characterization involved atomic force microscopy (AFM) and electrochemical analysis. Optimization of the construction and response of the immunosensor was achieved at incubation times of 6 h for anti-CA 15-3, 1 h for bovine serum albumin, and 1 h for interaction with CA 15-3. The sensor displays a linear range between 2 and 16 U/mL, with a sensitivity of 0.012 μA/U mL⁻¹, a limit of detection (LOD) of 0.56 U/mL, and a limit of quantification (LOQ) of 1.88 U/mL. The interfering substances minimally affected the signal, with 4.94% response variation, and the reproducibility of the immunosensor demonstrated a relative standard deviation (RSD) of 5.65%. The sensor successfully determined the CA 15-3 concentration in human serum and saliva, demonstrating its potential for clinical analysis.

Acute Myocardial Infarction (AMI) is a critical cardiac condition that poses a substantial threat to myocardial function. Expedient diagnosis of AMI is paramount and relies on serological assays for rapid and accurate quantification of relevant biomarkers. Electrochemical sensors have emerged as promising candidates for this application, owing to their accessibility, operational simplicity, and high specificity. In this study, we developed a paper-based electrochemical immunosensor to detect cardiac troponin I in serum and saliva specimens. The electrode was fabricated using screen-printing technology with photographic paper as the substrate, employing graphite-based ink, nail polish, and acetone as the solvent. A quasi-reference electrode was constructed using silver powder-based ink, nail polish, and acetone. The immunosensor was prepared by modifying the working electrode with gold nanoparticles (AuNP) functionalized with cardiac troponin I antibodies (anti-cTnI) and bovine serum albumin (BSA). This modified electrode was subsequently used to detect the troponin I antigen. The analyses were performed in 0.1 mol L⁻¹ phosphate buffer medium, pH 7.00, in the presence of 5.0 mmol L⁻¹ of the potassium ferrocyanide probe. The immunosensor exhibited a sensitivity of 0.006 µA/fg mL⁻¹, a limit of detection of 9.83 fg mL⁻¹, and a limit of quantification of 32.79 fg mL⁻¹. Specificity studies conducted in the presence of other macromolecules demonstrated minimal interference, with relative standard deviations (RSD) below 5.00%, indicating a specific interaction with troponin I. Furthermore, the immunosensor demonstrated excellent reproducibility and stability. Upon application to serum and saliva samples, the immunosensor presented recoveries of approximately 99–105%, suggesting its potential applicability in clinical analyses.

The gold nanoparticle (AuNP) was synthesized using the well-known Turkevich method. This article explains didactic step-by-step of the synthesis, showing pictures of the entire process, including a well-explained mechanism and characterization study. The synthesis includes the reduction of NaAuCl4 using sodium citrate in high temperature (around 90 ⁰C). The two main mechanisms used to explain the AuNP synthesis via Turkevich method were explained. The first mechanism considerate that a nanowire intermediary and the other proposed that aggregate intermediates are not formed at any time during the synthesis. The materials (NaAuCl4 and AuNP) were characterized by UV-Vis, SEM, AFM, XRD and DLS. The UV-Vis exhibit an absorption maximum in 521 nm because the surface plasmon resonance (SPR) absorption band of AuNP. The SEM images of NaAuCl4 presents crystals with cubic shapes, while the AuNP presents average particle size of about 16-25 nm and particles that appear mainly spherical. To confirm the particles shapes, the AFM was conducted and it was possible to observe clearly the individual spherical nanoparticles. The XRD of AuNP showed the four main characteristic peaks for silver corresponding to (111), (200), (220) and (311) planes, confirming the cubic (FCC) silver. The DLS presented the average particle size of 3.3 ± 0.9 nm and the polydispersity index (PDI) of 0.574. In summary, the AuNP was synthesized using a simple and fast method. The result was a spherical and ultrasmall particle, that can be used is several applications.