1n sodium hydroxide

2′-deoxycytidine-5′-monophosphate
(dCMP), 2′-deoxyguanosine-5′-monophosphate (dGMP), 2′-deoxyadenosine-5′-monophopshate
(dAMP), 2′-deoxythmidine-5′-monophosphate (dTMP), sodium
chloride, 1N hydrochloric acid, and 1N sodium hydroxide were purchased
from Sigma-Aldrich. Anatase TiO₂ particles were purchased
from US Research Nanomaterials stock number #US3498. All chemicals
were used without additional modification or purification. For clarity,
nucleotides have three main functional groups, the nitrogenous ring,
the ribose sugar ring, and the phosphate group. The nitrogenous ring
and ribose are defined as the nucleosides. The nucleoside with the
phosphate group is the nucleotide.

Cell aggregates were generated using non-adhesive poly(2-hydroxyethyl methacrylate; pHEMA, Sigma Aldrich)-coated plates. pHEMA was dissolved in 95% ethanol to a final concentration of 3 mg/ml and sterile-filtered. The pHEMA solution was added to cell culture plates, which were left to dry overnight at room temperature. Cells were seeded in the pHEMA-coated plates at a density of 5 × 10⁴ cells/cm² for aggregate generation. Collagen gels were prepared by diluting collagen type I (rat tail, Corning) with cell culture media to a final concentration of 2.5 mg/ml and neutralizing with 1 N sodium hydroxide (Sigma Aldrich). Cells were mixed with the diluted collagen solution at a density of 5 × 10⁶ cells/ml, and 100 μl of the solution was plated in 35 mm dishes and gelled at 37 °C for 30 min. In order to harvest cells for characterization, cell aggregates were singularized by StemPro Accutase (Gibco) for 15 min with gentle pipetting, and collagen gels were dissociated by collagenase type 1 (Sigma Aldrich) for 15 min at 37 °C.