Multifunctional green pigment with UV protection based on capping agent-engineered Cr-doped hydroxyapatite nanoparticles

BB creams are multifunctional cosmetic formulations that combine tone correction, skin refinement, and sun protection. Common limitations of conventional UV filters and pigments include low photostability, environmental toxicity and skin penetration. To address these limitations, we developed chromium-doped hydroxyapatite (Cr-HAp) nanoparticles (with an effective dopant concentration ∼10 at%) modified with succinic, tartaric, and etidronic acids. Nanoparticles were synthesised via co-precipitation and hydrothermal treatment, with structural and surface modifications confirmed by XRD, FTIR spectroscopy, TEM, and XPS. The use of organic acids enabled a controlled oriented attachment (OA) process, allowing for precise morphology control with nanoparticle lengths ranging from short rods (∼15 nm) to elongated needles (up to 150 nm). Cr³⁺ incorporation into the hydroxyapatite lattice produced stable green pigments with adjustable CIELab colour coordinates (a* from ∼-12 to -8 and b* from ∼0 to 2.5) and lightness (70 to 85). Optical characterisation demonstrated effective UV protection through absorption across the UVB spectrum (280-320 nm), which was modulated by the specific capping agent and synthesis conditions. A prototype BB cream incorporating 1.23% w/w of modified Cr-HAp nanoparticles demonstrated their suitability as multifunctional cosmetic ingredients, successfully combining UV shielding, colour correction, and environmentally conscious formulation design.