materials Article Near Infrared-Activated Dye-Linked ZnO Nanoparticles Release Reactive Oxygen Species for Potential Use in Photodynamic Therapy Jaspreet Singh Nagi 1 , Kenneth Skorenko 2 , William Bernier 2,3 , Wayne E. Jones 3,4 and Amber L. Doiron 1, * 1 Department of Electrical and Biomedical Engineering, University of Vermont, Burlington, VT 05405, USA; Jaspreet-Singh.Nagi@uvm.edu 2 ChromaNanoTech LLC, Binghamton, NY 13902, USA; kenneth@chromananotech.com (K.S.); wbernier@chromananotech.com (W.B.) 3 Department of Chemistry, Binghamton University (SUNY), Binghamton, NY 13902, USA; wayne.jones@unh.edu 4 Provost and Vice President for Academic Affairs, University of New Hampshire, Durham, NH 03824, USA * Correspondence: amber.doiron@uvm.edu; Tel.: +1-802-656-3382 Received: 14 October 2019; Accepted: 12 December 2019; Published: 18 December 2019   Abstract: Novel dye-linked zinc oxide nanoparticles (NPs) hold potential as photosensitizers for biomedical applications due to their excellent thermal- and photo-stability. The particles produced reactive oxygen species (ROS) upon irradiation with 850 nm near infrared (NIR) light in a concentration- and time-dependent manner. Upon irradiation, ROS detected in vitro in human umbilical vein endothelial cells (HUVEC) and human carcinoma MCF7 cells positively correlated with particle concentration and interestingly, ROS detected in MCF7 was higher than in HUVEC. Preferential cytotoxicity was also exhibited by the NPs as cell killing was higher in MCF7 than in HUVEC. In the absence of irradiation, dye-linked ZnO particles minimally affected the viability of cell (HUVEC) at low concentrations (<30 μg/mL), but viability significantly decreased at higher particle concentrations, suggesting a need for particle surface modification with poly (ethylene glycol) (PEG) for improved biocompatibility. The presence of PEG on particles after dialysis was indicated by an increase in size, an increase in zeta potential towards neutral, and spectroscopy results. Cell viability was improved in the absence of irradiation when cells were exposed to PEG-coated, dye-linked ZnO particles compared to non-surface modified particles. The present study shows that there is potential for biological application of dye-linked ZnO particles in photodynamic therapy. Keywords: cytotoxicity; poly (ethylene glycol); cell viability; surface modification; photosensitizer 1. Introduction Reactive oxygen species (ROS) can oxidize DNA, proteins, and lipids, causing oxidative stress which can lead to permanent cellular Damage [1–3]. While ROS are produced by cells under normal physiological conditions as byproducts during electron transport, excess ROS react with critical cellular components and can lead to cell death [4–6]. Photodynamic therapy (PDT) requires the production of these cytotoxic reactive species after activation of the photosensitizer by wavelength-matched light to initiate cell killing [7–9]. After excitement of the photosensitizer upon light irradiation, electrons return to the ground state and release the excess energy, which reacts with available molecular oxygen, generating ROS such as singlet oxygen ( 1 O 2 ), hydrogen peroxide, or superoxide anions [10–13]. PDT can be harnessed to kill unwanted cells, such as those in cancerous tumors. ROS are additionally Materials 2020, 13, 17; doi:10.3390/ma13010017 www.mdpi.com/journal/materials