Submicron-sized phosphors based on hexagonal rare earth oxycarbonate for near-infrared excitation and emission

Neodymium-doped rare-earth oxycarbonates, Ln2O2CO3: x mol% Nd3+ (Ln = La, Gd, and Y, 0 ≤ x ≤ 5), were synthesized by a simple flux method using a 0.476Li2CO3–0.270Na2CO3–0.254K2CO3 eutectic mixture. The suitability of these oxycarbonates as stable, nontoxic bioimaging phosphors that exhibit near-infrared (NIR) emission under NIR excitation was investigated. Gd2O2CO3: x mol% Nd3+ and Y2O2CO3: x mol% Nd3+ were obtained in single-phase form with a hexagonal type-II structure. The particle sizes of the Gd2O2CO3: x mol% Nd3+ and Y2O2CO3: x mol% Nd3+ were on a submicron scale suitable for bioimaging, but that of La2O2CO3: x mol% Nd3+ was outside the suitable range for bioimaging. Gd2O2CO3: x mol% Nd3+ and Y2O2CO3: x mol% Nd3+ showed characteristic emissions corresponding to the f–f transitions of Nd3+. The strongest emission peak was observed at around 1064 nm under excitation at 823 nm in the samples containing 2 mol% of Nd3+. The emission intensity of Gd2O2CO3: 2 mol% Nd3+ was 1.25 times higher than that of Y2O2CO3: 2 mol% Nd3+. The emission intensity of both phosphors was also significantly higher than that of the indocyanine green in practical use as a bio-imaging reagent.