Supporting Information

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S-1

Supporting Information

In Situ Functionalized Fluorescent WS2-QDs as

Sensitive and Selective Probe for Fe

3+

and a

Detailed Study of Its Fluorescence Quenching

Vijay K. Singha, Himanshu Mishraa, Rashid Alib, Sima Umraoc, Rajesh Srivastavaa, Shiju Abrahamf_{, Arvind Misra}b_{, Vidya Nand Singh}e_{, Hirdyesh Mishra}d_{, R.S.Tiwari}a*_{, Anchal}

Srivastavaa*

a_{Department of Physics, Institute of Science, Banaras Hindu University, Varanasi– 221005,India.} b_{Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi–} 221005,India.

c_{Department of Mechanical Engineering, School of Mechanical and Aerospace Engineering,} Korea Advanced Institute of Technology (KAIST), Dajeon-34141, South Korea.

d_{Department of Physics (MMV), Banaras Hindu University, Varanasi– 221005, India.} e_{CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi– 110012, India.} f_{The Zuckerberg Institute for Water Research, The Jacob Blaustein Institutes for Desert} Research, Ben-Gurion University of the Negev, Sede Boqer-8499000, Israel.

*Corresponding authors:

E-Mail ID: [email protected] (Prof. Anchal Srivastava), [email protected](Prof. R.S. Tiwari), Phone No.: +91-9453203122, +91-9415991746

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S-2 FTIR Analysis:

Fourier transform infrared (FTIR) spectroscopy is performed to investigate the presence of functional groups over the surface of WS2-QDs. FTIR spectra of f-WS2-QDs is shown in Figure S1. C-H bending and C-O stretching modes were located around ~ 509 cm-1_{, ~ 615 cm}-1_, ~ 1124 cm-1 and ~ 1318 cm-1. FTIR band around ~ 1400 cm-1 correspond to the S=O vibrations of sulphate group. Moreover, NH out of plane and in plane NH2 vibrations of amide group was located around ~ 1600 cm-1. N=C vibration was located around ~ 2064 cm-1. A band centered around ~ 3500 cm-1 indicates the stretching mode (O-H stretch) of hydrogen of hydroxyl group.

Figure S1. FTIR spectrum of colloidal f-WS2-QDs.

Quantum Yield:

The quantum yields of f-WS2-QDs and f-WS2-QDs-Fe3+ were estimated with respect to anthracene (Φ = 0.3) as standard in ethanol solution by using following equation.

𝑄 = 𝑄𝑅 𝐼 𝐼𝑅 𝑂𝐷𝑅 𝑂𝐷 𝑛2 𝑛𝑅2 (1) 500 1000 1500 2000 2500 3000 3500 4000 T ran smi ttan ce (a. u .) Wavenumber (cm-1) O -H Stretch Dimer O -H N=C NH out of plane & NH 2 in plane S=O C -O Stre tch C -O Stre tch CH b e nd CH b end

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S-3

Where, Q is the quantum yield. QR is the quantum yield of reference sample I is the integrated intensity, OD is the optical density, and n is the refractive index. The subscript R refers to the reference fluorophore of known quantum yield.

Table S1. Quantum yield of f-WS2-QDs and f-WS2-QDs+Fe3+ (in H2O) using anthracene as a reference.

Sample Integrated

Emission Intensity (I)

Absorbance (A) Refractive Index of Solvent (η) Quantum Yield (Ф) Anthracene 286101 0.0875 1.36 30% (Known) f-WS2-QDs 49830.31489 0.108138 1.33 ~4.04% f-WS2-QDs + Fe3+ 1514.70648 0.132497 1.33 ~0.1% Limit of Detection:

A linear regression curve obtained from the plot of the normalized fluorescence intensities of f-WS2-QDs with Fe3+ versus Log [Fe3+] is shown in Figure below. This plot has been used for calculation of limit of detection and it was found to be as low as 1.32 µM.

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S-4

Figure S2. Showing the linear calibration plot for estimation of detection limit. Linearity of Sensor:

Figure S3 showing the linear relatinoship between the relative fluoroscence intensity (on log scale) and the concentration of Fe3+. 1, 2 The regression equation is ln(F0/F) = -0.24 + 0.078 [Q], (r = 0.997), where F0 and F are the fluoroscence intensities of f-WS2-QDs in absence and presence of Fe3+ ions.

-5.7

-5.4

-5.1

-4.8

-4.5

-4.2

0.0

0.2

0.4

0.6

0.8

1.0 Y = 0.67354x + 3.9601

R

2 = 0.9966

(

I

- I

m

in

)

/

(

I m

ax

- I

m

in

)

Log

[

Fe

3+

]

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S-5

Figure S3. Showing the variation in fluoroscence intensity of f-WS2-QDs on log scale versus quencher (Fe3+) concentraion.

Absorption spectra of f-WS2-QDs with varying concentrations of Fe3+ ions:

Figure S4 showing the absorption spectra of f-WS2-QDs solution with gradual addition of Fe3+ ions into the solution. From here it can be observed that intensities of absorption bands at 333 nm and red edge of the absorption spectra increases with the addition of Fe3+. The increment in absorption bands intensities clearly reveals the formation of dark complexes in ground state (static quenching). 0 10 20 30 40 50 60 1.0 2.7 7.4 20.1 54.6 ln (F 0 /F ) [Q] (M)

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S-6

Figure S4. Showing the absorption spectra of f-WS2-QDs with gradual addition of quencher (Fe3+) from 0 to 60 µM.

Time resolved photoluminescence data:

TRPL spectra of f-WS2-QDs have been recorded with a laser excitation of 375 nm. TRPL spectrum obtained is fitted for an exponential function as follows:

𝑦 = 𝛼₁𝑒𝑥𝑝 (− 𝑡 𝜏1) + 𝛼2𝑒𝑥𝑝 (− 𝑡 𝜏2) + 𝛼3𝑒𝑥𝑝 (− 𝑡 𝜏3) (2)

Table S2. Life time corresponding to each tri-exponential decays (τ1, τ2, and τ3).

Conc of Fe3+ (µM) τ1 α1(%) τ2 α2(%) τ3 α3(%) χ 2 0 0.74 31 3.48 41 10.53 27 1.169 6.6 0.63 28 3.22 41 10.09 30 1.098 13.2 0.61 40 3.32 35 10.06 23 1.112 19.8 0.53 35 3.34 39 10.35 26 1.07 26.4 0.52 37 3.18 36 10.18 25 1.177 33 0.49 42 3.31 35 10.37 22 1.118 39.6 0.47 41 3.27 33 9.77 25 1.261 46.2 0.45 39 3.25 33 9.94 28 1.112 52.8 0.45 51 3.19 28 9.87 20 1.115 59.4 0.42 53 3.2 28 10.26 19 1.161 300 400 500 600 A b so rb an ce (a. u .) Wavelength (nm) 60 μM 0 μM Fe3+

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S-7 Reference:

1. Liu, Z.; Liu, S.; Yin, P.; He, Y. Fluorescence Enhancement of CdTe/CdS Quantum Dots by Coupling of Glyphosate and its Application for Sensitive Detection of Copper Ion. Anal.

Chim. Acta 2012, 745, 78-84.

2. Zhu, X.; Zhang, Z.; Xue, Z.; Huang, C.; Shan, Y.; Liu, C.; Qin, X.; Yang, W.; Chen, X.; Wang, T. Understanding the Selective Detection of Fe3+_{Based on Graphene Quantum Dots as} Fluorescent Probes: The K sp of a Metal Hydroxide-Assisted Mechanism. Anal. Chem. 2017, 89, 12054-12058.