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Biosensors and Bioelectronics
journal homepage: www.elsevier.com/locate/bios
Profuse color-evolution-based fluorescent test paper sensor for rapid
and visual monitoring of endogenous Cu
2+
in human urine
Yueqing Cai, Junhui You, Zhengyi You, Fang Dong, Shuhu Du
⁎
, Liying Zhang
⁎
School of Pharmacy, Nanjing Medical University, Nanjing, Jiangsu 211166, China
ARTICLE INFO
Keywords:
Fluorescent test paper
Sensor
Tricolor probe
Visual detection
Cu
2+
ABSTRACT
The fluorescent paper for colorimetric detection of metal ions has been widely fabricated using various sensing
probes, but it still remains an elusive task to design a test paper with multicolor variation with target dosages for
accurate determination. Herein, we report a profuse color-evolution-based fluorescent test paper sensor for
rapid and visual monitoring of Cu
2+
in human urine by printing tricolor probe onto filter paper. The tricolor
probe consists of blue-emission carbon dots (bCDs), green-emission quantum dots (gQDs) and red-emission
quantum dots (rQDs), which is based on the principle that the fl uorescence of gQDs and rQDs are
simultaneously quenched by Cu
2+
, whereas the bCDs as the photostable internal standard is insensitive to
Cu
2+
. Upon the addition of different amounts of Cu
2+
, the ratiometric fluorescence intensity of the tricolor probe
continuously varied, leading to color changes from shallow pink to blue with a detection limit of 1.3 nM. When
the tricolor probe solution was printed onto a sheet of filter paper, as-obtained test paper displayed a more
profuse color evolution from shallow pink to light salmon to dark orange to olive drab to dark olive green to slate
blue to royal blue and to final dark blue with the increase of Cu
2+
concentration compared with dual-color
probe-based test paper, and dosage scale as low as 6.0 nM was clearly discriminated. The sensing test paper is
simple, rapid and inexpensive, and serves as a visual platform for ultrasensitive monitoring of endogenous Cu
2+
in human urine.
1. Introduction
Recently, the quantification of metal ions is extensively performed
by using conventional techniques, including atomic absorption spectro-
scopy (NG and Garner, 1993), mass spectrometry (Richardson, 2001),
inductively coupled plasma-mass spectrometry (ICP-MS) (Bings et al.,
2006), and so on. However, these techniques usually require ponderous
instruments, the sophisticated sampling and professional operation by
well-trained personnel, making it difficult to on-site monitor metal ions
in environments and biological fluids. Therefore, there is an urgent
demand for developing a simple, economical and portable method for
in vitro and in vivo metal ions’ assay, motivating considerable
researchers to construct new miniature chemical sensors.
Semiconductor, such as quantum dots (QDs) or carbon dots (CDs), is
a promising optical label for chemo/biosensing applications since it
offers distinct advantages. These include (1) good optical properties, (2)
good photochemical stability, (3) long fluorescence lifetime, and (4) good
water solubility (Wang and Guo, 2009). Thus, it is a suitable candidate
for fluorescent probe, which can be linked a recognition element to
generate fluorescent “turn on”, “turn off” or “ratiometric” response.
Unfortunately, it still relies on fluorescent instrument, for example,
fluorescent spectrometer or confocal microscope. To solve this problem,
fluorescent test paper has been developed because it possesses an
excellent and unparalleled merit, that is, its visualization capability for
the detection of target analyte by the naked eye with the aid of a portable
ultraviolet (UV) lamp. Generally, the fluorescent test paper is prepared
by printing various fluorescent probes onto filter paper or microporous
membrane. To date, fluorescent test paper utilizing a fluorescent single-
color probe has been reported in many previous literatures, such as
graphene oxide paper for the detection of peptide, protein, and DNA
(Mei and Zhang, 2012), QDs paper for the assay of catechol and glucose
(Yuan et al., 2012) and CDs paper for the analysis of mercuric ions (Yuan
et al., 2014). But they can only exhibit the variation of fluorescence
brightness by either “turn on” or “turn o
ff” mo
de with analytes, which
greatly limits their quantitative capability. More recently, test paper
based on dual-emission fluorescent probe has been fabricated for the
assay of trinitrotoluene by QDs@SiO
2
-QDs paper (Zhang et al., 2011),
the detection of sulfur dioxide by ratiometric QDs@SiO
2
-QDs paper
(Yan et al., 2015), the monitoring of pesticides by coumarin-3-carboxylic
acid-QDs@SiO
2
paper (Li et al., 2015), the quantification of glucose by
http://dx.doi.org/10.1016/j.bios.2017.07.072
Received 23 May 2017; Received in revised form 17 July 2017; Accepted 29 July 2017
⁎
Corresponding authors.
E-mail addresses: shuhudu@njmu.edu.cn (S. Du), zly@njmu.edu.cn (L. Zhang).
Biosensors and Bioelectronics 99 (2018) 332–337
Available online 31 July 2017
0956-5663/ © 2017 Elsevier B.V. All rights reserved.
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