Upconversion-luminescent hydrogel optical probe
for in situ dopamine monitoring
BINGQIAN ZHOU,
1
JINGJING GUO,
1,2
CHANGXI YANG,
1
AND LINGJIE KONG
1,
*
1
State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University,
Beijing 100084, China
2
e-mail: guojj018@tsinghua.edu.cn
*Corresponding author: konglj@tsinghua.edu.cn
Received 20 July 2020; revised 31 August 2020; accepted 8 September 2020; posted 8 September 2020 (Doc. ID 403223);
published 30 October 2020
Dopamine (DA), as a neurotransmitter in human brain, plays a crucial role in reward motivation and motor
control. An improper level of DA can be associated with neurological disorders such as schizophrenia and
Parkinson’s disease. To quantify DA, optical DA sensors have emerged as an attractive platform due to their
capability of high-precision and label-free measurement, and immunity to electromagnetic interference.
However, the lack of selectivity, limited biocompatibility, and complex fabrication processes are challenges that
hinder their clinical applications. Here, we report a soft and biocompatible luminescent hydrogel optical sensor
capable of recognizing and quantifying DA with a simple and compact interrogation setup. The sensor is made of
a hydrogel optical fiber (HOF) incorporated with upconversion nanoparticles (UCNPs). DA molecules are de-
tected through the luminescence energy transfer (LET) between the UCNPs and the oxidation products of DA,
while the light-guiding HOF enables both excitation and emission collection of the UCNPs. The hydrogel sensor
provides an optical readout that shows a linear response up to 200 μmol/L with a detection limit as low as
83.6 nmol/L. Our results show that the UCNP-based hydrogel sensor holds great promise of serving as a soft
and biocompatible probe for monitoring DA in situ.
© 2020 Chinese Laser Press
https://doi.org/10.1364/PRJ.403223
1. INTRODUCTION
Dopamine (DA) is a central monoamine neurotransmitter in-
volved in the regulation of a wide range of complex processes,
including normal brain function, emotions, muscle movement,
and hormones [1–3]. High levels of DA in the brain are respon-
sible for reward and pleasurable feelings, whereas its deficiency
can lead to stress, depression, and motor disorders. Impaired
DA transmission is associated with many neurolo gical diseases,
including epilepsy, drug addiction, memory loss, schizophrenia,
Parkinson’s disease, attention deficit hyperactivity disorder, and
psychiatric problems [4,5]. In addition, recent studies have
shown that DA concentrations in tumor tissues are usually
lower than those in normal tissues [6 ]. Therefore, the develop-
ment of efficient approaches for quantitative measurement of
DA should facilitate diagnosis and treatment of neurodegener-
ative disorders and cancers in clinics.
To date, several methods have been developed for DA de-
tection, including microdialysis [7], liquid chromatography
(HPLC) [8], electrochemistry [9–13], colorimetry, and lumi-
nescent spectroscopy techniques [14–20]. Microdialysis and
HPLC have long been the gold standard for quantitative
measurement of DA but suffer from the limitations of compli-
cated instrumentation and high cost. Fast-scan cyclic voltam-
metry (FSCV), one of the most popular electrochemical
methods, has been successfully utilized to quantify DA with
high temporal resolution and sensitivity that, however, has
difficulties in distinguishing DA from other competing species
of similar oxidation potential. Despite the recent progress
achieved in improving the selectivity of electrochemical meth-
ods, they are still limited to a few electroactive species [21].
Luminescent and colorimetric probes such as upconversion
nanoparticles (UCNPs) and quantum dots (QDs) have also
shown great potential in detecting DA because of their high
sensitivity and selectivity [22–25]. However, these methods re-
quire the nanoprobes to be dispersed in the analyte sample for
the subsequent fluorescence analysis, which suffers from sample
contamination and is susceptible to environmental interfer-
ence. An attractive alternative for the design of DA sensing
probes is to use functionalized optical fibers due to their minia-
turized size, low cost, remote sensing capability, and electro-
magnetic interference (EMI) immunity [26–30]. For example,
Agrawal et al. demonstrated an optical fiber probe functional-
ized with silver nanoparticles and polyethylene glycol (PEG) for
DA detection based on the localized surface plasmon resonance
(LSPR) effect [26]. Kim et al. proposed a miniaturized and
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Vol. 8, No. 11 / November 2020 / Photonics Resea rch
Research Article
2327-9125/20/111800-08 Journal © 2020 Chinese Laser Press