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Organic Dye-Modified Upconversion Nanoparticle as A Multi-Channel Probe to Detect Cu2+ in Living Cells Bin Gu, Minan Ye, Lina Nie, Yuan Fang, Zilong Wang, Xiao Zhang, Hua Zhang, Yi Zhou, and Qichun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13351 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

Organic Dye-Modified Upconversion Nanoparticle as A Multi-Channel Probe to Detect Cu2+ in Living Cells ⊥

Bin Gu,‡† Minan Ye,⊥†Lina Nie,‡ Yuan Fang, Zilong Wang,‡ Xiao Zhang,‡ Hua Zhang,*‡ Yi Zhou,*§ Qichun Zhang*‡§ ‡

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

Avenue, 639798, Singapore. ⊥

College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816,

P. R. China §

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical

Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore.

KEYWORDS: multi-channel, upconversion, emission, electrochemistry, bio-imaging

ACS Paragon Plus Environment

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ABSTRACT: Developing an inorganic-organic hybrid probe to more accurately detect ions in living systems is very challenging but highly desirable. Here we combined upconversion nanoparticles with electrically-active ferrocene group to detect Cu2+ in living cells. The asprepared probe displays three different signal changes in absorption, emission, and electrochemical behavior respectively during detecting Cu2+ ions. Moreover, this new probe has been demonstrated to show high stability and adaptability. In addition, bio-imaging test reveals that this probe is suitable for detecting and visualizing Cu2+ in A549 cells with low cytotoxicity.


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Introduction Copper ion is one of main trace metal cations in human body, which plays a key role in metabolism. Its disequilibrium in human body would cause several severe diseases such as Menkes disease1 and Wilson’s disease.2 Therefore, sensitively and efficiently monitoring copper ions in both environment and human body is of great importance. Recently, lanthanide-doped upconversion nanoparticles (UCNPs) have been demonstrated to be a promising probe because UCNPs can successively absorb two or more low energy photons via intermediate long-lived energy states, and emit a high energy photon, which is known as the conversion of near-infrared (NIR) excitation into visible or UV emission,3-4 with a large antiStokes shift on spectrum.5-6 The as-reported research results have already proven that UCNPs possess many advantages including no auto-fluorescence from bio-tissue,7-10 deep penetration depth,8,

11-12

chemical stability,7,

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long lifetime (milliseconds)5,

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and less harmful to bio-

samples.15-18 Continuing to work on this research direction, scientists have successfully developed a novel detection strategy, namely, loading chromophore on the UCNPs to detect ions or bio-signals through luminescence resonance energy transfer (LRET) process. In LRET systems, UCNPs can absorb long-wavelength photons and emit short-wavelength photons (visible range), which can be further absorbed by specific chromophores. In such cases, the emission from UCNPs would become invisible. If targeted ions or bio-signals can interact with chromophores to cause the change of their absorption, the emission from UCNPs can be turned on or off, which can be used to detect ions or bio-signals. In fact, several impressed probes based on LRET process have been reported recently. Li and coworkers reported cyanine-modified UCNPs for methylmercury detection,19 Chang and coworkers discovered dye-assembled UCNPs for zinc ion detection in vitro and in vivo,20 Liu and coworkers created manganese-dioxide-


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loaded nano-sheets for intracellular glutathione detection,21 Chen and coworkers demonstrated a time-resolved bio-probe to detect avidin,22 Liu and coworkers used NaYF4-based sandwich structure for the detection of calcium ion

23

and silver nano-cluster based UCNPs for bio-thiol

detection,24 Qu and coworkers designed hyaluronic-acid-based UCNPs for the detection and imaging of reactive oxygen species (ROS).25 Some other rhodamine-based UCNP probes were also reported.26-29 Our group have published some progresses based on similar strategy, including cyanine-modified UCNPs for sensing endogenous hydrogen peroxide signaling in vivo,30 rhodamine-modified UCNPs for the detection of hypochlorous acid in living cells,31 organic hybrid UCNPs for the detection and bio-imaging of hydrogen sulfide in mouse model,15 and thiazole-derivative-modified UCNPs for the recognition of mercury ion.32 Though these probes are effective, they are all single-channel-based and easy to be affected by environment. In order to address this issue, we would like to develop a new type of probes with multi-channel responses because these probes provide higher adaptability, better selectivity and self-calibration ability. Here, we demonstrated a new strategy by integrating the signals from absorption, UCNPs-based emission, and electrochemistry into one system. Our as-prepared probe shows the improved stability and adaptability. To the best of our knowledge, this is the first time to import electrochemical signals into UCNPs-based probe. This as-designed probe consists of a lanthanide-doped UCNP (NaYF4:20%Yb,1.8%Er,0.5%Tm) core with silica as a hydrophilic shell, followed by loading copper-ion-responsive chromophores to form the final probe RB-FC-UCNPs.

Results and Discussion


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We employed NaYF4 doped with Yb3+, Er3+ and Tm3+ as UCNP, which is a common combination of sensitizers and activators. Transmission electron microscopy (TEM) image (Figure S4) shows that there were no significant changes in shape and crystallinity after the modification with silica layer. Both OA-UCNP and silica-coated-UCNP had an average diameter within 15-20 nm, and no evident aggregation was observed. Powder X-ray diffraction (PXRD) peaks of the UCNPs correlated well with the hexagonal structure of NaYF4, proving the high purity of the UCNPs (Figure S5). Our target chromophore RB-FC has a weak absorbance in solvent. However, after the addition of Cu2+, the as-formed complex has a maximum absorption peak at 558 nm (Figure S6a), which perfectly matches the upconversion luminescence (UCL) emission of 2H11/2→4I15/2, 4S3/2→4I15/2 transitions of Er3+ (Scheme 1A). In addition, at different pH, fluorescence intensity at 554 nm of Cu2+ in EtOH/H2O (1:1, v/v) has also been investigated (Fig. S6b). The emission bands of UCNPs overlap with the absorption band of the chromophore, indicating that UCNPs could transfer energy to RB-FC, and suggesting that RB-FC-UCNPs is suitable for triggering LRET process (Scheme 1B). We also imported a ferrocene group into RB-FC because electrochemical signal produced by FeIII/FeII redox couple could be combined with optical signal33. In this study, we employed the absorption peak at 558 nm (A558) as one detection channel, the UCL intensity ratio of 540 nm to 654 nm (I540/I654) as second one, and electrically-active ferrocene as third channel. RB-FC-UCNPs did not show strong absorption peak, while with the presence of Cu2+, we observed a maximum absorption peak at 558 nm (Figure 1A), corresponding to a color change to red, which was in agreement with pure RB-FC. Using A558 as a detection signal, the limit of detection was measured to be 0.11µM (Figure S7).


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For UCL test, under 980 nm excitation, RB-FC-UCNPs show four emission bands at 514−534 nm, 534−560 nm, 635−680 nm and 775-825 nm, attributed to 2H11/2→4I15/2, 4S3/2→4I15/2 and 4

F9/2→4I15/2 transitions of Er3+, and 3H4→3H6 of Tm3+, respectively. After the addition of Cu2+,

the RB-FC-Cu2+ complex has strong absorption at 558 nm and the LRET process will be turned on. Two emission bands were gradually quenched, and we could only find the other two: 635−680 nm and 775-825 nm (Figure 1C). Therefore, it is reasonable to suggest that the degree of quenching could be used to calculate the concentration of Cu2+. We also noticed that when the emission intensity gradually decreased, the band at 534−560 nm decreased much faster than the band at 514−534 nm (Figure 1D). This was because the absorption spectrum of RB-FC-Cu2+ complex overlapped more with the band at 534−560 nm than with the band at 514−534 nm, and the emitted photons within the band at 534−560 nm were much easier to be absorbed by RB-FCCu2+ complex. This result would further confirm that the quench was caused by LRET process. Since the other two emission bands (635−680 nm and 775-825 nm) were not involved in the LRET process and their intensity should not be affected by the addition of Cu2+, we can employ these two bands as internal reference standards, and the intensity of the detected channel could be compared with these references, which could make our result more accurate. Here we employed UCL intensity ratio of 540 nm to 654 nm (I540/I654) as detection signal, and the limit of detection was measured to be 5.95 µM (Figure S8). Though the limit of detection is higher than that of absorption signal, in real sample test, the detection based on UCL signal will be more promising because of the high signal-to-noise ratio. As an ion probe, high selectivity plays a key role. To verify the selectivity of RB-FC-UCNPs, we tested some other metal ions such as alkali metal (K+, Na+), alkali earth metal (Ca2+, Mg2+) and some transition metal (Ag+, Mn2+). We calculated intensity ratio of 540 nm to 654 nm. With the


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presence of Cu2+, the intensity ratio was much lower than other metal ions (Figure S9), suggesting excellent selectivity. Probes containing ferrocene groups will show remarkable potential shift of FeIII/FeII redox couple before and after the interaction with analyte. Here we conducted electrochemical test based on ferrocene group. Without Cu2+, the oxidation peak of RB-FC-UCNPs is 0.548V (Figure 2A (upper)). When gradually increasing the concentration of Cu2+ to 140 µM, both oxidation peak and reduction peak decreased due to the formation of RB-FC-Cu2+ complex, and the oxidation peak decreased to 0.377V (Figure 2A (bottom)). The oxidation peak had a linear relationship with the concentration of Cu2+ (Figure 2B), and the limit of detection was measured to be 10.0 µM. Meanwhile, we also calculated the limit of detection based on reduction peak to be 8.2 µM (Figure S10), which was similar to the oxidation one. This similarity proves the stability of electrochemical detection. Here we list detection results of UCNPs-based probe for Cu2+ detection from 2014 (Table 1). Compared these results, they are almost on the same level. The highlight of this paper is developing a three-channel probe by importing electrochemical signals into the detection. Though our probe could be further improved, we still hope to emphasize on this new strategy. Before conducting the bio-imaging test in living cells, the cytotoxicity has been first investigated using the methyl thiazolyl tetrazolium (MTT) assay (Figure S11). Following the incubation of RB-FC-UCNPs for 24h, the probe exhibited small cytotoxicity to A549 cells. The cellular viability of A549 cells was still higher than 80% even at a high concentration of 500 µg ml-1, proving that the RB-FC-UCNPs is a biocompatible probe suitable for bio-imaging applications.


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To monitor intracellular Cu2+ through RB-FC-UCNPs, we conducted a laser-scanning upconversion luminescence microscopy (LSUCLM) test (Figure 3). Under 980 nm laser, ratiometric (G/R) of control group was 10.82±0.45, and LRET process did not happen yet. After the addition of Cu2+, the ratiometric (G/R) of test group decreased to 5.96±0.37, indicating that LRET process was triggered, and green emission band had been largely quenched. This result suggested that the probe could be applied to monitor Cu2+ in living cells.

Conclusions In summary, we imported electrochemical signals into UCNPs-based probe for Cu2+ detection. Through absorption signal, UCNPs-based emission signal, and electrochemical signal, this multichannel detection system shows high stability and high selectivity. In addition, this strategy has been applied in bio-samples and we monitored Cu2+ in A549 cells with low cytotoxicity. The new strategy that importing electrochemical signal into UCNPs-based detection should be helpful to illuminate the important roles of Cu2+ in human health and environment, as well as to provide a promising future for further detection and bio-imaging of other ions and molecules.


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Figure 1. (A) Absorption spectrum of 0.12 mg ml-1 RB-FC-UCNPs in DMSO with increasing Cu2+ concentration from 4-20 µM. (B) The absorption at 558 nm increased with the addition of Cu2+. (C) UCL spectrum of 1.2 mg ml-1 RB-FC-UCNPs in DMSO with increasing Cu2+ concentration from 0.0-0.4 mM. (D) The ratio of UCL intensity using 654 nm as reference standard decreased with the addition of Cu2+.


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Figure2. (A) Cyclic voltammogram of RB-FC-UCNPs before (upper) and after (bottom) the addition of Cu2+. (B) Oxidation peak values decreased when adding Cu2+.


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Figure 3. Ratiometric UCL images in A549 cells. 0.5 mg ml-1RB-FC-UCNPswere added to control group (A to C) and test group (D to F). After 45 minutes, test group were treated with 200 µM Cu2+. Emission was collected by both the green channel at 500-560 nm (A and D) and red channel at 600-700 nm (B and E). (C and F) Ratiometric UCL images with ratio of green to red channels.


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Scheme 1. (A) Emission of UCNP and absorption of RB-FC + Cu2+ overlapped at 540 nm band. (B) Structure of RB-FC-UCNP and illustration of LRET process from UCNPs to RB-FC.

Table 1. Limit of detection of UCNPs-based probe for Cu2+ detection from 2014.

This Paper

Literature Report

Signal Type

Limit of Detection

Reference

Absorption-based

0.11µM

___

Emission-based

5.95µM

___

Oxidation-peak-based

10.0 µM

___

Reduction-peak-based

8.2 µM

___

___

2.16 µM

34

___

0.82 µM

35

ASSOCIATED CONTENT


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Supporting Information The

following

1

13

H-NMR,

files

are

available

free

of

charge.

C-NMR, TEM, powder-XRD, absorption spectrum, absorption and emission

intensity ratio, selectivity test result, MTT assay, and how to prepare OA-UCNPs and RB-FCUCNPs. AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

(Hua

Zhang),

[email protected]

(Yi

Zhou),

[email protected] (Qichun Zhang) Author Contributions †

These authors contributed equally.

ACKNOWLEDGMENT Q. Z. acknowledges financial support from Academic Research Fund Tier 1 (RG13/15 and RG 8/16) from the Ministry of Education, Singapore. Q.Z. also thanks the support from Open Project of State Key Laboratory of Supramolecular Structure and Materials (Grant number: sklssm201733), Jilin University, China. Y. Z. acknowledges financial support from Natural Science Foundation of Jiangsu Province (Grants BK20170996) and Jiangsu Science and Technology Support Program (BE20150023).


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