Spectroscopic System for Direct Lanthanide Photoluminescence Spectroscopy with Nanomolar Detection Limits

Abstract

A new spectroscopic system for direct photoluminescence of lanthanide ions (Ln(III)) through electronic transitions within the 4fⁿ manifold is described. The system is based on an injection seeded frequency tripled (λ = 355 nm) Nd:YAG pump laser coupled with a master oscillator power oscillator (MOPO). The MOPO delivers an average pulse energy of ⪝60 mJ/pulse, is continuously tunable from 425 to 690 nm (Signal) and 735 to 1800 nm (Idler) with a linewidth of <0.2 cm⁻¹, and has a pulse duration of 10–12 ns. Aqueous solutions containing two polyaminocarboxylate complexes, ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA), and Ln³⁺ aqua ion for several lanthanides including Eu(III), Tb(III), Dy(III), and Sm(III)) are used as steady-state and time-resolved photoluminescence standards. The versatility of the instrument is demonstrated by excitation scans over a broad visible range for aqueous solutions of complexes of Eu(III), Dy(III), Sm(III), and Tb(III). The Eu(III) excitation band (⁷F_o→⁵D_o) is recorded over a range of complex concentrations that are 1000–fold less than reported previously, including Eu(EDTA) (1.00 nM), Eu(DTPA) (1.00 nM), and Eu(III) aqua ion (50.0 nM). Emission spectra are recorded in the visible range for Ln(III) complexes at pH 6.5 and 1.00 mM. Excited-state lifetimes for the standards were constant as a function of concentration from 10.0 nM to 1.00 mM for Eu(EDTA) and Eu(DTPA) and from 100 nM to 1.00 mM for Eu(III) aqua ion. Photoluminescence lifetimes in H₂O and D₂O are recorded and used to calculate the number of bound water molecules for all complexes.