We synthesized the persistent luminescent ZGO:Cr 3+ hydrothermally to yield cubic phase nanocrystals with diameters of 10 ± 2nm (Figure ) and a zeta potential of 34 ± 1.1 mV -making the particles easily dispersed in water. The luminescence spectrum in Figure at 696nm is the emission of the spin-forbidden 2 E à 4 A2 transition of Cr 3+ (5). As expected, the ZGO:Cr 3+ NPs have 3 excitation bands (Figure ) -254 nm, 420 nm, and 580 nm representing the d-d transitions of Cr 3+ (5). Using a pulse excitation of 420 nm, the Cr 3+ emission peak showed a multiexponential decay and an effective luminescence lifetime (teff) of 531 µs using equation S1. The luminescence decay at 696nm proceeds well into the millisecond range after either a 420 nm (Figure ) or 266 nm (Figure ) pulse excitation, deeming the ZGO:Cr 3+ ready for the next steps towards incorporation into our optode-based persistent luminescence sensing approach. In most biological applications, modifications are needed to link recognition sites and/or stabilizing agents to the ZGO:Cr 3+ surface for sensing or better aqueous stability . However, to incorporate our ZGO:Cr 3+ PLNPs they must have a hydrophobic surface to be retained in the organic phase of the optode-based nanosensor. Thus, the raw ZGO:Cr 3+ NPs were modified by condensation of trimethoxy(octyl) silane to the hydroxide surface of ZGO:Cr 3+ NPs (Figure ) to yield ZGO:Cr 3+ @TMOS that are stable in nonpolar environments like the optode matrix. The modification was confirmed with Fourier transform infrared (FT-IR) spectroscopy where the peaks in the siloxane region (1090 cm -1 , 1053 cm -1 , 1033 cm -1 , and 1015 cm -1 ) represent the Si-O-Si stretching, indicating successful condensation and the bands at 2958 cm -1 , 2923 cm -1 , and 2855 cm -1 represent the C(sp 3 )-H bonds of the octyl silane attachment protruding from the particle the surface (Figure ). After ligand addition, the ZGO:Cr 3+ @TMOS can be dispersed in a variety of organic solvents (THF, chloroform, toluene) for further use. Flash nanoprecipitation is a nanoparticle fabrication approach that can encapsulate a range of materials into polymer nanoparticles . Here, we used FNP to fabricate persistent luminescent nanocomposites of ZGO:Cr 3+ @TMOS and poly styrene-b-poly(ethylene) oxide (PS-b-PEO)(Figure ). Their hydrophobic core is a mixture of ZGO:Cr 3+ @TMOS and PS while the PEO coat provides aqueous stability; though, other block copolymers can be used for fabrication (Table ). Before the incorporation of sensing components, we used the ZGO:Cr 3+ @TMOS@PS-b-PEO (shown in Figure ) to show that the autofluorescence of fetal bovine serum (FBS) can be completely omitted from our luminescence measurements (Figure ). The FBS is strongly autofluorescent with a broad peak at 500 nm which overlaps with the ZGO:Cr 3+ emission unless a time-resolved acquisition approach is used.