Each amino acid was prepared as a 0.05 M solution in deionized water before zinc acetate dihydrate was added at the same concentration. The mixtures were stirred for five minutes and heated to 90 °C for 30 minutes. The lysine bath became cloudy during this treatment, whereas solutions containing glutamic acid, serine, cystine or histidine remained transparent. Sodium carbonate was therefore added to those four preparations at 0.05 M, and heating continued for another 30 minutes at 90 °C. This distinction is important because the study did not include a zinc acetate–sodium carbonate control synthesis. The results show how the five amino-acid systems behaved relative to one another under the selected conditions, but they do not establish that an amino-acid route outperforms conventional alkali-mediated ZnO synthesis.

X-ray diffraction revealed that the glutamic acid-, serine-, lysine- and cystine-assisted products contained crystalline ZnO with reflections assigned to the (100), (002) and (101) planes of hexagonal wurtzite ZnO. No peaks attributable to metallic zinc, zinc hydroxide or other crystalline zinc phases were detected in those four samples. The (101) reflection was strongest in each crystalline preparation. The ratio of (002) polar-plane intensity to (100) nonpolar-plane intensity followed the order lysine, glutamic acid, serine and cystine, indicating differences in crystallographic orientation. However, these intensity ratios do not by themselves prove facet-selective adsorption, because peak intensity can also vary with crystallite size, preferred orientation, sample preparation and instrumental conditions.

Diffraction-peak widths allowed the researchers to estimate the dimensions of coherently diffracting crystalline domains. Values calculated from the (101) reflection were 4.00 nanometres for cystine, 8.12 nanometres for glutamic acid, 13.06 nanometres for serine and 30.73 nanometres for lysine. Lysine therefore produced the largest estimated coherent domains and comparatively narrow peaks, while cystine produced the smallest domains and broadest peaks. Peak broadening can reflect smaller crystallites, lattice strain, structural disorder or a combination of these factors. Calculated lattice parameters and c/a ratios fell within the expected range for wurtzite ZnO. Lower estimated microstrain and dislocation density were also reported for the lysine-derived material, but the diffraction data did not identify the molecular processes responsible for these differences.

Measurements of the suspensions before annealing revealed a pronounced ranking in colloidal behavior. Dynamic light scattering gave intensity-weighted hydrodynamic Z-average diameters of approximately 15 nanometres for lysine, 51 nanometres for serine, 130 nanometres for glutamic acid, 230 nanometres for cystine and 1,190 nanometres for histidine. The lysine sample also had the highest positive zeta potential at 37.5 ± 2 millivolts, followed by serine at 28.3 ± 3 millivolts, glutamic acid at 24.1 ± 2 millivolts, cystine at 16.7 ± 3 millivolts and histidine at 13.1 ± 3 millivolts. This sequence indicates progressively weaker electrostatic stabilization across the listed preparations under the measurement conditions. It cannot be attributed solely to amino-acid surface chemistry, because the final dispersion pH was not measured and ZnO surface charge depends strongly on pH relative to its isoelectric point.

Dynamic light scattering also describes suspension behavior rather than the dry dimensions of individual particles. Its hydrodynamic diameters include particle cores, hydration layers, adsorbed molecules and weakly associated aggregates. Transmission electron microscopy was instead performed after the recovered material had been freeze-dried and annealed at 250 °C for six hours, so the images reflect synthesis combined with thermal processing. Serine yielded predominantly spherical particles with an average diameter of 10 ± 3 nanometres. Glutamic acid produced elliptical to nearly spherical particles averaging 50 ± 5 nanometres, while lysine generated predominantly rod-shaped structures about 55 ± 5 nanometres wide. Much larger rectangular forms appeared in the cystine preparation, with an edge length of approximately 650 ± 25 nanometres, and the histidine preparation showed apparent hexagonal structures averaging about 600 ± 30 nanometres.

Because annealing can dehydrate intermediates, remove organic residues, promote crystallization and cause limited particle growth or coalescence, the observed shapes cannot be attributed exclusively to the amino acids. The limited number of micrographs also prevents a comprehensive statistical evaluation of the particle-size distributions. The histidine result illustrates why shape and phase must be interpreted separately: its particles had an apparent hexagonal outline, but its powder lacked the characteristic diffraction peaks of wurtzite ZnO, indicating that crystalline ZnO was not detected under the investigated conditions. Its large hydrodynamic diameter, low zeta potential and broad optical absorption were consistent with strong aggregation. Fourier-transform infrared spectroscopy identified hydroxyl and carboxylate-related bands, but the product could have been an amorphous zinc-containing phase, residual zinc hydroxide species, a zinc–histidine coordination compound or another noncrystalline intermediate.

Optical measurements performed before annealing showed absorption edges at 362 nanometres for lysine, 365 nanometres for serine, 367 nanometres for glutamic acid, 369 nanometres for histidine and 371 nanometres for cystine. These corresponded to calculated optical band gaps of 3.42, 3.39, 3.37, 3.36 and 3.34 electronvolts, respectively. Lysine and serine produced narrower absorption features, while histidine and cystine showed broader, red-shifted features consistent with their greater aggregation and lower crystallinity. The band-gap sequence broadly paralleled the structural results, with the more ordered and dispersed systems having larger apparent gaps. Nevertheless, the shifts cannot be explained by particle size alone: morphology, coherent crystallite dimensions, surface states, aggregation and defect-related electronic states may all contribute. The 3.36-electronvolt value for histidine cannot be presented as the confirmed band gap of crystalline ZnO because X-ray diffraction did not establish that phase. No defect-sensitive measurements were performed, so the shifts cannot be assigned to oxygen vacancies or other particular defects.

Fourier-transform infrared spectra retained evidence of amino-acid-associated functionality after washing, freeze-drying and annealing. Broad bands between 3,255 and 3,435 inverse centimetres were assigned to hydroxyl stretching from surface groups and physically adsorbed water, while bands in the 1,400–1,600 inverse-centimetre region were attributed to symmetric and asymmetric carboxylate vibrations. Their persistence shows that at least some amino-acid-derived groups remained associated with the products, although lower band intensities than in unannealed materials suggested that much of the physically adsorbed organic material had been removed. The cystine sample additionally exhibited a band near 643 inverse centimetres, consistent with Zn–S interactions or other sulfur-containing surface species. Histidine showed hydroxyl and carboxylate-related features, but these did not establish a specific coordination geometry.

Together, the measurements are consistent with amino acids acting as coordination- and growth-modifying agents rather than conventional reducing agents during ZnO formation. ZnO synthesis proceeds through hydrolysis, precipitated zinc hydroxide intermediates and dehydration into ZnO, allowing amino-acid functional groups to affect zinc coordination, precursor availability, nucleation, crystal growth, aggregation and surface stabilization. The comparison nevertheless cannot isolate amino-acid molecular structure from accompanying changes in pH and other solution-chemistry effects. Lysine’s carbonate-free formation, large coherent crystallite estimate, rod-shaped annealed particles and strongest measured colloidal stability were encouraging within this experimental framework, but they do not prove a specific molecular pathway. The work provides a comparative basis for selecting biomolecular additives in ZnO synthesis, while application performance, nanoparticle leaching, residual zinc species, ecotoxicity and life-cycle effects remain unexamined.

Subject of Research: Comparative effects of five polar amino acids on ZnO nanoparticle formation and physicochemical properties

Article Title: Influence of polar amino acid-assisted synthesis on the formation and growth of ZnO nanoparticles

Article References: Ghosh, S., Goswami, P., & Rana, S. (2026). Influence of polar amino acid-assisted synthesis on the formation and growth of ZnO nanoparticles. Cleaner Engineering and Technology, 35, Article 101340. https://doi.org/10.1016/j.clet.2026.101340

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101340

Keywords: ZnO nanoparticles, polar amino acids, biomolecule-assisted synthesis, wurtzite ZnO, crystal growth, colloidal stability, particle morphology, optical band gap, nanoparticle nucleation, zinc acetate, sodium carbonate, Influence