Gold nanoparticles that shift from red to blue on contact with caprylic acid, one of the main fatty acids in bovine milk, form the basis of an exploratory study aimed at building a simple, low-cost colorimetric sensor for milk fat. The work, published in the journal Bioscience Nanotechnology, systematically compared how citrate-stabilized gold nanoparticles respond to palmitic, oleic, and caprylic acids, the three fatty acids that together account for roughly 98 percent of the fat in milk. Milk itself contains about 3 to 6 percent fat, and its fat content is a key quality parameter that directly influences the price paid to dairy producers, the nutritional profile of products, and their industrial performance. Conventional fat determination methods such as the Gerber method, which isolates fat after decomposing proteins with sulfuric acid and centrifugation, are effective but involve hazardous reagents, multiple processing steps, and specialized laboratory infrastructure, while chromatographic and spectroscopic alternatives demand high financial investment that makes routine use in the dairy industry difficult.
The research team synthesized gold nanoparticles through citrate-mediated reduction of chloroauric acid, a route known as the Turkevich method, in which sodium citrate acts simultaneously as reducing and stabilizing agent. The formation of nanoparticles was confirmed by a visual transition from pale yellow to the characteristic wine-red color of colloidal gold and by the appearance of a localized surface plasmon resonance absorption band centered at approximately 525 nanometers. Dynamic light scattering measurements revealed an average hydrodynamic diameter of 1.2 plus or minus 0.2 nanometers, a polydispersity index of 0.351 plus or minus 0.185, and a zeta potential of minus 10.30 plus or minus 3.74 millivolts, indicating moderate colloidal stability. The authors note that although the Turkevich method is commonly associated with particles in the 10 to 30 nanometer range, size depends strongly on synthesis conditions such as the citrate-to-gold ratio, reagent addition rate, stirring intensity, reaction temperature, and pH, and rapid nucleation with restricted growth can yield ultrasmall particles or gold clusters.
Because long-chain fatty acids such as palmitic acid, a saturated C16 compound, and oleic acid, a monounsaturated C18 compound, are strongly hydrophobic and disperse poorly in water, the researchers used the nonionic surfactant Triton X-100 to solubilize them. Fatty acids were added to concentrated Triton X-100, heated in a water bath at 100 degrees Celsius for 20 minutes to aid solubilization, and then diluted with distilled water. In the colorimetric assays, varying volumes of these fatty acid solutions and the surfactant were added to the gold nanoparticle suspension, producing fatty acid concentrations in the reaction medium ranging from 9.02 times 10 to the minus 3 to 72.16 times 10 to the minus 3 grams per milliliter. A mixture of the three fatty acids was also tested, with its composition set according to the relative proportions typically found in milk fat: 50 percent palmitic acid, 45 percent oleic acid, and 5 percent caprylic acid by mass. A control sample containing only Triton X-100 was prepared under the same conditions.
The central finding is that the colorimetric response of the gold nanoparticles depends strongly on the physicochemical properties of the fatty acids, especially carbon chain length and solubility. Visual inspection showed concentration-dependent color changes for all tested fatty acids, but the most pronounced response came from caprylic acid, the short-chain C8 fatty acid, which produced a clear transition from red to blue tones. Palmitic and oleic acids induced more subtle changes, with the suspensions predominantly retaining reddish to purple hues across the evaluated concentration range, and these changes became evident mainly at higher concentrations. The fatty acid mixture produced a measurable but intermediate chromatic behavior relative to the individual components, and statistical analysis of the color values showed that the mixture behaved similarly to palmitic acid, with no significant differences between their red or blue color value parameters.
Quantitative image analysis supported these observations. Digital images of the assay vials were captured with a smartphone under controlled lighting, and fixed regions of interest were processed to extract red, green, and blue intensity levels. For the nanoparticle control, the mean red intensity was 152.73 plus or minus 2.37, the green 53.42 plus or minus 2.86, and the blue 74.37 plus or minus 2.90, reflecting the characteristic red coloration. With palmitic acid, the red intensity decreased in a non-linear fashion, reaching a minimum of 103.85 plus or minus 1.00 at a concentration of 36.08 times 10 to the minus 3 grams per milliliter, while the blue level showed only minor variations. Oleic acid produced a similar non-linear decrease in red intensity, although at the highest concentration the red value rose again to 138.71 plus or minus 10.52, and its blue intensity increased to a maximum of 96.03 plus or minus 13.86. Caprylic acid induced the most pronounced changes of all, driving the red intensity down to a minimum of 30.93 plus or minus 2.73 at 15.78 times 10 to the minus 3 grams per milliliter, and at the two highest concentrations the blue intensity exceeded the red, consistent with the visible red-to-blue transition.
Ultraviolet-visible absorption spectroscopy corroborated the image-based results. For palmitic acid, oleic acid, and the fatty acid mixture, the plasmon band remained centered near 525 nanometers with no significant wavelength displacement, although absorbance intensity at that region varied with fatty acid concentration in a non-linear way, indicating subtle modifications of the local dielectric environment or partial surface interactions that did not progress to extensive aggregation. Caprylic acid, by contrast, caused a pronounced red shift of the plasmon band from approximately 525 nanometers to about 630 nanometers at higher concentrations, a displacement that corresponds to plasmonic coupling resulting from nanoparticle aggregation. The spectra also showed an elevated baseline at longer wavelengths and irregular features, attributed to light scattering from aggregates, likely involving both Rayleigh and Mie scattering depending on aggregate size. The authors deliberately analyzed unprocessed colloidal dispersions without filtration or centrifugation, because preliminary experiments showed that both procedures partially sedimented or retained the nanoparticles and compromised the evaluation of the nanoparticle-fatty acid interactions.
Dynamic light scattering measurements taken after fatty acid addition provided direct evidence of aggregation. At the lowest tested concentration, caprylic acid already promoted a substantial increase in particle size to 28.39 nanometers compared with the as-synthesized 1.2 nanometer particles, whereas palmitic acid induced only minor changes, to 3.29 nanometers. At the highest concentration, pronounced aggregation occurred, particularly for caprylic acid at 162.93 nanometers and oleic acid at 188.7 nanometers, accompanied by elevated polydispersity values indicating highly polydisperse aggregates. The reduction in the magnitude of the zeta potential, especially for caprylic acid at minus 5.09 millivolts, suggests decreased electrostatic stabilization after fatty acid interaction. The authors propose that the interaction involves a combination of hydrophobic interactions between alkyl chains and micellar domains, van der Waals forces, and electrostatic or coordination interactions between deprotonated carboxylate groups and the gold surface, with the surfactant forming micellar structures that expose the carboxylate groups to the aqueous phase.
The reason caprylic acid outperforms its longer-chain counterparts lies in its chemistry. Because of its shorter hydrocarbon chain, caprylic acid exhibits lower lipophilicity and greater effective solubility in the aqueous-micellar environment, which favors more efficient access of its carboxylate group to the negatively charged nanoparticle surface. This facilitates stronger surface interaction, reduces interparticle distance, and enhances plasmonic coupling, producing the larger spectral shifts and the visible color change. Palmitic and oleic acids, with their longer hydrocarbon chains, experience greater steric hindrance and reduced surface accessibility, limiting effective adsorption and preventing close approach to the nanoparticles. The low absolute zeta potential of the synthesized particles, attributed to adsorbed citrate ions, means the system is susceptible to aggregation upon chemical perturbation, a property the authors describe as particularly relevant for sensing, since aggregation reduces interparticle distance and promotes the plasmonic coupling that generates visible color changes.
To describe the concentration trends quantitatively, the researchers fitted polynomial or logarithmic regressions to the color value data, since the responses did not follow strictly linear behavior, a nonlinearity they attribute mainly to turbidity and aggregation processes that influence light scattering, plasmonic coupling, and digital image acquisition. For palmitic acid, the red color value followed a fourth-order polynomial model with an R squared of 0.8767, while the blue value followed a second-order polynomial with an R squared of 0.8398. For oleic acid, the red component was described by a third-order polynomial with an R squared of 0.9369 and the blue by a second-order polynomial with an R squared of 0.9717. Caprylic acid yielded a third-order polynomial for the red value with an R squared of 0.8868 and a fourth-order polynomial for the blue with an R squared of 0.9779, and the mixture showed a third-order polynomial for red with an R squared of 0.8953 and a logarithmic model for blue with an R squared of 0.9236. Statistical significance among samples was assessed by one-way analysis of variance followed by Tukey’s post hoc test at a significance level of p below 0.05.
The authors position this work as an initial and essential step toward gold nanoparticle-based colorimetric sensors rather than a finished analytical method. The combined use of RGB image analysis and ultraviolet-visible spectroscopy proved effective at capturing both subtle and pronounced color changes, and the sensitivity of the red channel makes it a promising parameter for quantitative evaluation. Because gold nanoparticles are low in toxicity, biocompatible, inert and stable against oxidation, and easily adapted to different purposes, the researchers argue they are well suited to colorimetric techniques that could support portable and on-site analyses in dairy quality control. The study was conducted with synthetic fatty acid samples under controlled laboratory conditions, and the authors emphasize that the system has the potential to become an innovative alternative to the conventional analytical methods currently used in dairy quality control, offering simplicity, rapid visual response, low operational cost, and minimal instrumentation requirements as the future development of a milk fat quantification nanosensor proceeds.
Subject of Research: Colorimetric interaction mechanisms between gold nanoparticles and the main fatty acids in bovine milk under controlled synthetic conditions
Article Title: Initial and exploratory of interaction mechanisms between AuNPs and fatty acids under controlled synthetic conditions for future colorimetric sensing applications
Article References: Ribeiro, P. A., Monteiro, A. L. S. A., Lofrano, R. C. Z., & Santos, I. J. B. (2026). Initial and exploratory of interaction mechanisms between AuNPs and fatty acids under controlled synthetic conditions for future colorimetric sensing applications. Bioscience Nanotechnology, 2(1), Article 11. https://doi.org/10.1186/s44331-026-00021-x
Image Credits: AI Generated
DOI: 10.1186/s44331-026-00021-x
Keywords: gold nanoparticles, fatty acids, colorimetric sensing, milk fat, caprylic acid, palmitic acid, oleic acid, localized surface plasmon resonance, RGB image analysis, UV-Vis spectroscopy, dynamic light scattering, dairy quality control
News Source: Gregory Coleman. (October 30, 2026). Gold nanoparticles turn blue in the presence of milk fatty acids, pointing toward low-cost fat sensing. Scienmag.
