**A Cu(II) Indicator Platform Based on Cu(II) Induced Swelling that Changes the Extent of Fluorescein Self-Quenching**

#### **Feifei Wang** †**, Roy P. Planalp and W. Rudolf Seitz \***

Department of Chemistry, University of New Hampshire, Durham, NH 03824, USA;

fsq5@wildcats.unh.edu (F.W.); Roy.Planalp@unh.edu (R.P.P.)

**\*** Correspondence: rudi.seitz@unh.edu; Tel.: +1-603-862-2408

† Current address: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 1H9, Canada; fwang3@ualberta.ca; Guangzhou Medical University, Guangzhou 511436, China.

Received: 26 June 2019; Accepted: 13 November 2019; Published: 25 November 2019

**Abstract:** In this study, we established a new fluorescent indicator platform. The responsive element consists of poly(N-isopropylacrylamide) nanospheres that include small percentages of fluorescein and a ligand, anilinodiacetate (phenylIDA). Nanosphere diameters were determined to be in the range from 50 to 90 nm by scanning electron microscopy. They were entrapped in a polyacrylamide gel to prevent nanosphere aggregation. At pH 6, the ligand is negatively charged in the absence of metal ions. Charge-charge repulsion causes the nanosphere to swell. Dynamic light scattering measurements show that these nanospheres do not shrink and aggregate at high temperature. Cu(II) binding neutralizes the charge causing the particles to shrink. This brings fluoresceins closer together, increasing the degree of self-quenching. The intensity decreases by 30% as Cu(II) concentration increases. To rule out the possibility that the observed decrease in intensity was due to Cu(II) quenching of fluorescence, we also added Zn(II) and observed a decrease in intensity. This approach can be adapted to sense di fferent metal ions and di fferent concentrations of Cu(II) by changing the ligand.

**Keywords:** self-quenching; pNIPAM; cross-linked nanoparticles; copper; PA gel
