Luminescence encoding of polymer microbeads with organic dyes and semiconductor quantum dots during polymerization

Abstract

Luminescence-encoded microbeads are important tools for many applications in the life and material sciences that utilize luminescence detection as well as multiplexing and barcoding strategies. The preparation of such beads often involves the staining of premanufactured beads with molecular luminophores using simple swelling procedures or surface functionalization with layer-by-layer (LbL) techniques. Alternatively, these luminophores are sterically incorporated during the polymerization reaction yielding the polymer beads. The favorable optical properties of semiconductor quantum dots (QDs), which present broadly excitable, size-tunable, narrow emission bands and low photobleaching sensitivity, triggered the preparation of beads stained with QDs. However, the colloidal nature and the surface chemistry of these QDs, which largely controls their luminescence properties, introduce new challenges to bead encoding that have been barely systematically assessed. To establish a straightforward approach for the bead encoding with QDs with minimized loss in luminescence, we systematically assessed the incorporation of oleic acid/oleylamine-stabilized CdSe/CdS-core/shell-QDs into 0.5–2.5 µm-sized polystyrene (PS) microspheres by a simple dispersion polymerization synthesis that was first optimized with the organic dye Nile Red. Parameters addressed for the preparation of luminophore-encoded beads include the use of a polymer-compatible ligand such as benzyldimethyloctadecylammonium chloride (OBDAC) for the QDs, and crosslinking to prevent luminophore leakage. The physico-chemical and optical properties of the resulting beads were investigated with electron microscopy, dynamic light scattering, optical spectroscopy, and fluorescence microscopy. Particle size distribution, fluorescence quantum yield of the encapsulated QDs, and QD leaking stability were used as measures for bead quality. The derived optimized bead encoding procedure enables the reproducible preparation of bright PS microbeads encoded with organic dyes as well as with CdSe/CdS-QDs. Although these beads show a reduced photoluminescence quantum yield compared to the initially very strongly luminescent QDs, with values of about 35%, their photoluminescence quantum yield is nevertheless still moderate.

Influence of Nanoparticle Encapsulation and Encoding on the Surface Chemistry of Polymer Carrier Beads

“…For QD incorporation during bead formation, the QDs were dispersed in styrene and added during the polymerization procedure. 31 Thereby, the QDs are exposed to harsh reaction conditions, such as an elevated temperature and the presence of radicals, which can also affect their uorescence properties. For the bead swelling procedure, rst unstained, carboxylated microbeads were synthesized with a polymerization procedure previously established and adapted for the surface functionalization with COOH groups.…”

“…For the bead swelling procedure, rst unstained, carboxylated microbeads were synthesized with a polymerization procedure previously established and adapted for the surface functionalization with COOH groups. 31 These unstained, pristine beads were also employed as control or blank to determine the in uence of QD staining on the bead surface FGs. Bead encoding by a swelling and shrinking step was utilized by us before to encode differently sized and surface functionalized polymer beads with organic luminophores and sensor dyes.…”

“…Synthesis of CdSe/CdS-core/shell-QDs. The CdSe/CdS-QDs with a core/shell-architecture in hexane were prepared according to a previously described procedure 31 [21][22][23] The synthesis is described in detail in the Supplementary Information (SI).…”

“…Synthesis of unstained and QD-encoded PSMPs via a polymerization procedure (route i). The PSMPs were synthesized following a previously described polymerization procedure, 31 modi ed considering two other procedures reported by Kimura et al 43 and Nirmalananthan-Budau et al (regarding COOH functionalization). 38 Typically, 400 mg of PVP were dissolved in 45 mL of a mixture of ethanol and water (9:1).…”

“…In a previous study on QD encoding of PSMPs, utilizing very similar polymerization conditions with additional crosslinking of the polymer network with divinylbenzene, yet not a surface functionalization with carboxyl groups, we observed a QD accumulation in the PSMP core region (no signi cant agglomeration of the QDs). 31 This indicates that the presence of acrylic acid with its COOH groups, and possibly also the absence of the crosslinker, prevent QD migration into the PSMP core. This nding can also explain why the zeta potential of the PSMPs is modi ed in the case of the swelling procedure, leading to an increase in zeta potential, contrary to those prepared by the polymerization procedure.…”