A covalent organic framework bearing thioether pendant arms for selective detection and recovery of Au from ultra-low concentration aqueous solution

Article abstract of DOI:10.1039/c8cc05369c

A fluorescent covalent organic framework (COF), featuring precise distribution of thioether pendant arms inside the cavity, was designed. The thioether-functionalized COF exhibits selective sensing and capture of Au ions at ultra-trace levels in water with high sensitivity, selectivity and adsorption capacity, which makes it an excellent candidate for selective detection and recovery of Au.

Full text of DOI:10.1039/c8cc05369c

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A covalent organic framework bearing thioether pendant arms for
selective detection and recovery of Au from ultra-low
concentration aqueous solution
Received 10th July 2018,
Accepted 30th July 2018
Zhiming Zhou,ab Wanfu Zhong,ab Kaixun Cui,ab Zanyong Zhuang,ab Lingyun Li,ab Liuyi Li,*ab Jinhong
Bib and Yan Yu*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
A fluorescent covalent organic framework (COF), featuring precise characteristics of the COFs,6d selective sensing of Au ions may
distribution of thioether pendant arms inside the cavity, was be achieved.
designed. The thioether-functionalized COF exhibits selective
Herein, we report a design of thioether-functionalized COF
sensing and capture of Au ions at ultra-trace levels in water with (TTB-COF) for selective sensing and recovery of low
high sensitivity, selectivity and adsorption capacity, which makes it concentration Au in water (Scheme 1). TTB-COF exhibits the
an excellent candidate for selective detection and recovery of Au.
function of selectively, qualitatively, and quantitatively
recognizing Au over other metal ions at low concentration levels
by significant luminescence quenching through the strong and
selective affinity of sulfur atoms for Au ions.
The recovery of Au from electronic wastes is extremely both
important from environmental and economic point of views,
because of its limited availability and high price. Much effort has
been devoted to develop alternative methods to the highly toxic
cyanide heap leaching process1 to recovery of Au from high-
concentration solution.2 However, little work has been done on
selective detection and separation of Au from ultra-low
concentration aqueous solution, as it requires sensitive
response and specific affinity to Au.
TTB-COF bearing thioether pendant arms as binding sites for
Au8 was solvothermally synthesized through the reversible
imine condensation reaction of 2,5-bis(2-(ethylthio)ethoxy)
terephthalohydrazide (BETH) and 1,3,5-triformylbenzene (TFB).
Fourier transform infrared spectroscopy (FT-IR) spectrum of
TTB-COF (Fig. S1, ESI†) showed a new characteristic vibrational
band appeared at 1620 cm−1,9 revealing the formation of the
imine bonds via the successful condensation of BETH and TFB.
Solid-state 13C NMR spectroscopy further supports the
proposed chemical structures of TTB-COF. The formation of the
imine bond was established by a characteristic resonance signal
Covalent organic frameworks (COFs) are one of new
crystalline porous materials with well-defined structures.3 The
structure tunability and permanent porosity make COFs
a
promising materials platform for functional design.4
Furthermore, the extended π-conjugation in COFs endows
them with fluorescent property,5 endowing them emerging
candidates for sensing applications.6 We envision that
introduction of Au-binding sites inside the cavity of fluorescent
COFs may endow them with an ability to selectively detect and
recover low concentration Au. Such COFs would have specific
affinity of the binding sites in COFs to Au ions, providing a great
potential for selective adsorption of low concentration Au from
water.7 Another possible advantage of such a COF-based
recovery system is that through the establishment of a
relationship between the metal ions and luminescence
a.College of Materials Science and Engineering, Fuzhou University, Minhou,Fujian
350108, China. E-mail address: lyli@fzu.edu.cn; yuyan@fzu.edu.cn
b.Key Laboratory of Eco-materials Advanced Technology (Fuzhou University),
Fuzhou, Fujian 350108, China.
Scheme 1 Schematic representation of the synthesis of TTB-COF and its application in
selective detection and capture of Au ions.
†Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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relatively lower surface area than that of the theoretical value
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2929 m2 g-1) was mainly owing to the blockage of the surface
DOI: 10.1039/C8CC05369C
(
pore channels by small COF nanoparticles. The pore size
distribution reveals a homogeneous pore diameter of 1.4 nm
(Fig. S10, Inset, ESI†), which is consistent with the pore size
predicted from the crystal structures (1.7 nm). Scanning
electron microscopy (SEM) images show TTB-COF consists of
uniform microrods with a size of about 200 nm in diameter (Fig.
1c). The microrods are consisted from the primary nanosheets
with a few nanometers in thickness (Fig. S11, ESI†). The sheaf-
like TTB-COF hierarchical microrods were clearly observed by
TEM spectra (Fig. 1d and Fig. S12, ESI†). From the enlarged
image of one microrod, the microrod was constructed by many
interconnected nanosheets. Such a hierarchical structure is
would be beneficial for rapid mass transfer.12
UV-visible diffuse-reflectance spectrum (UV-Vis DRS) of TTB-
COF shows a broad absorption up to 700 nm in the solid state
(Fig. 1e). A maximum absorption at 400-550 nm is assigned to
the π–π* transitions of the conjugated ring systems. A large
bathochromic shift of TTB-COF from that of monomers is
attributed to the extended π-conjugation in TTB-COF. TTB-COF
is yellow in color (Inset in Fig. 1e), while it displays a bright
yellow photoluminescence when excited under 365 UV-light
(Inset in Fig. 1f). Upon excitation at 394 nm, TTB-COF in solid
state exhibits a maximum emission band at 540 nm with an
absolute quantum yield (ΦF) of 4.2% (Fig. 1f). Under the same
excitation wavelength, the monomers BETH and TFB display
weak fluorescence emission peaks at 513 and 470 nm,
respectively (Fig. S13, ESI†). The strong fluorescence in TTB-COF
was mainly attributed to the existence of a p-n heterojunction
between BETH and TFB units6b as well as the contorted
structure.6a, 6d It is worth mentioning that TTB-COF keeps its
high fluorescence in H2O, ethanol, acetonitrile, THF and DMF
(excitation wavelength λex= 394 nm, Fig. S14, ESI†), which is the
prerequisite for its potential application as fluorescent sensing
probes in solution.
Density functional theory (DFT) calculations (B3LYP/6-311g**)
of the optimized structure of building block shows that the
pendant thioether groups is electronically negative (Fig. S15,
ESI†), demonstrating that the thioether groups should
preferably coordinate to metal ions. The lowest unoccupied
molecular orbitals (LUMOs) and highest occupied molecular
orbitals (HOMOs) of the building block are similarly delocalized
over the entire conjugated structure and the band gap is
estimated to be 3.90 eV (Fig. S16, ESI†).
Fig. 1 (a) XRD patterns of experimental and simulated TTB-COF; (b) Top and side views
of the eclipsed structure of TTB-COF (red, S; blue, N; gray, C; purple, O); (c) SEM and (d)
TEM images of TTB-COF; (e) UV-vis absorption spectra of TTB-COF, BETH, and TFB. Inset:
photos of TTB-COF; (f) Fluorescence excitation (blue line) and emission (red line) spectra
of TTB-COF in solid state. Inset: TTB-COF under UV-light (365 nm).
at 160 ppm,10 while the signals at 15 and 36 ppm confirmed the
presence of thioether groups in the framework (Fig. S2, ESI†).11
Elemental analyses of the content of C, H, N and S in TTB-COF
were in good agreement with the theoretical values. In addition,
the TTB-COF could be synthesized conveniently in various
solvent systems (Fig. S3 and S4, ESI†), showing its facile
synthesis. TTB-COF was insoluble and stable in common organic
solvents and water (Fig. S5, ESI†), in particular, it can resist acid,
even in aqueous 1M HCl at 100 oC for 24 h (Fig. S6, ESI†).
Thermogravimetric analysis (TGA) shows that TTB-COF is stable
to 280 °C (Fig. S7, ESI†). Such chemical and thermal stability
makes TTB-COF attractive for use in metal ions recovery from
water.
The crystallinity of TTB-COF is determined by powder X-ray
diffraction (XRD) analyses. A set of intensive peaks at 2θ = 3.2o,
7.3o, and 25.7o were observed in the experimental XRD pattern
(Fig. 1a and Fig. S8), which are assignable to 100, 200, and 001
facets, respectively. To determine the packing manner of TTB-
COF, theoretical simulations are carried out using Materials
Studio software packages. The eclipsed stacking model (Fig. 1b)
is in remarkable agreement with the experimental XRD pattern
in terms of peak positions and relative intensities (Fig. 1a),
suggesting the 2D layers adopt eclipsed packing. By contrast,
the calculated XRD pattern for the staggered structure cannot
be consistent with the experimental XRD patterns (Fig. S9, ESI†).
The porosity of TTB-COF is studied by measuring N2
adsorption at 77 K. The BET surface area and pore volume are
36 m2 g−1 and 0.06 cm3 g−1, respectively (Fig. S10, ESI†). The
TTB-COF with fluorescent properties and the specific affinity
of pendant sulfur groups as well as the open pore structure can
be exploited as an ideal candidate for sensing and adsorption of
Au ions. As shown in Fig. 2a, the fluorescence intensity of TTB-
COF gradually decreased with increasing Au ions concentration,
accompanied by its color change under a UV lamp (with λex
=
365 nm) (Inset in Fig. 2a). The fluorescence intensity at 540 nm
of TTB-COF was significantly quenched with the addition of 0.83
μM of Au ions. 84% of the total fluorescence intensity of TTB-
COF was quenched with the addition of 10 μM of Au ions. The
comparison of the UV-vis spectrum of Au ions, fluorescence
excitation and emission spectra of TTB-COF demonstrates that
2 | Chem. Commun., 2018, 00, 1-3
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the used excitation wavelength (λex=394 nm) does not overlap of 10 ppm, TTB-COF still retains high capture efficiency for Au
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DOI: 10.1039/C8CC05369C
with the absorbance of Au ions (Fig. S17, ESI†), excluding the ions (Inset in Fig. 2f).
inner filter effect between Au ions and fluorescent TTB-COF.
An analogue of TTB-COF, COF-42 without thioether chain,10
The plot of fluorescence intensity versus different was used as the fluorescent probe for Au ions, however, only a
concentration of Au ions showed a linear relationship in the weak response to Au ions was observed under the identical
range of 1.0-10.0 μM (Fig. 2b). The quenching constant (Ksv) is conditions (Fig. S20 and S21, ESI†). Only 14% Au ions can be
4.46×105 M−1 from the linear Sterm-Volmer curve. The captured by COF-42, which was far less than the adsorption
detection limit is calculated to be 0.87 μM based on S/N = 3. The effect of TTB-COF (Fig. 3a and Fig. S22, ESI†). This indicates that
high quenching rate constant (2.84×1014 M-1 s-1) based on the the fluorescence quenching and the Au ions capture ability of
Ksv and the fluorescence lifetime of TTB-COF (1.57 ns) (Fig. S18, TTB-COF mainly result from the interactions between the sulfur
ESI†) indicates that the quenching efficiency is mainly ascribed groups in TTB-COF and Au ions, which was further verified by X-
to the static quenching effect. The maximum capture capacity ray photoelectron spectroscopy (XPS). In comparison with XPS
of TTB-COF is 560 mg g-1 (Fig. 2c). Even though the recovery spectra of TTB-COF, the binding energy peaks of N 1s and O 1s
experiments are carried out toward the low concentration Au, in TTB-COF after Au capture (designated Au/TTB-COF) were still
the sorption capacity is comparable with or higher than those remained at 399.6 eV, 530.9 eV, and532.9 eV (Fig. S23 and S24,
of the most efficient materials reported earlier (Table S1, ESI†), respectively, whereas the binding energy of S 2p3/2
ESI†).13 Moreover, the sensing of Au ions with TTB-COF can be upshifted from 163.2 to 165.7 eV (Fig. 3b), indicating the
carried out in water, and the detection limit is 1.39 μM (Fig. S19, specific and strong coordination interactions between S atoms
ESI†), suggesting its high potential for practical applications. The and Au ions in Au/TTB-COF.14 FTIR and XRD analyses of Au/TTB-
adsorption kinetics of TTB-COF toward Au ions in water showed COF revealed that no alternation in the crystalline and chemical
that over 98% of Au ions can be captured within 1 min (Fig. 2d). structures were observed, reflecting the high structural stability
of TTB-COF in Au recovery application (Fig. S25 and S26, ESI†).
SEM images of Au/TTB-COF show the sheaf-like hierarchical
microrods morphology is well remained (Fig. S27, ESI†). No
obvious Au particles were observed in TEM images (Fig. S28,
ESI†). Energy-dispersive spectroscopy (EDX) elemental mapping
images of Au/TTB-COF demonstrate that the uniform
distribution of the Au atoms within the framework is related to
the location of S elements (Fig. 3c and 3d), which is in
agreement with XPS results.
Fig. 2 (a) Fluorescence titration of TTB-COF in acetonitrile upon the gradual addition of
Au ions (λex = 394 nm), Inset: fluorescence images of quenching change of TTB-COF in
acetonitrile upon the addition of Au ions (under a UV lamp with λex = 365 nm); (b) Stern-
Volmer plots for the quenching of TTB-COF by Au ions; (c) Au ions adsorption isotherm
after 12 h; (d) Adsorption curves of various metal ions in aqueous solution. (e)
Fluorescence response of TTB-COF in the presence of different metal ions in H2O (λex
=
394 nm); (f) Capture efficiency for various metal ions. Inset: Capture efficiency of TTB-
COF for Au ions in presence of other competing metal ions after adsorption for 90
minutes.
Fig. 3 (a) Comparison of Au capture efficiency of TTB-COF and COF-42 in aqueous
solutions. (b) S 2p XPS spectra of TTB-COF and Au/TTB-COF. (c) TEM and (d) EDX
elemental mapping images of Au/TTB-COF.
Selectivity is an important factor for practical Au sensing and
recovery from water. As shown in Fig. 2e, Au ions exhibits an
extremely significant quenching effect on the luminescence
intensity of TTB-COF in water, while other transition metal ions
such as Fe3+, Ni2+, Co2+, Zn2+, and Cd2+ show relatively little
photoluminescence quenching. Only a small amount of the
investigated ions was adsorbed (Fig. 2f). In the mixture solution
of Au3+, Fe3+, Ni2+, Co2+, Zn2+, and Cd2+ ions at each concentration
To enrich Au ion from water at the low concentration,
Au/TTB-COF was treated with aqueous Na2S solution. A gradual
color change of was Au/TTB-COF observed from yellowish to
brown (Fig. 4a), which can be taken as a sign of the reduction
Au ions to nanoparticles (NPs).15 The emergence of plasmon
band at 590~675 nm in the UV-Vis DRS spectrum indicated the
formation of Au NPs in TTB-COF (designated Au(0)/TTB-COF)
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(Fig. 4b), which was confirmed through TEM analyses. TEM functional COFs for novel applications through rational design
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DOI: 10.1039/C8CC05369C
images of Au(0)/TTB-COF illustrate Au NPs were dispersed on at the atomic/molecular level.
the surface of TTB-COF (Fig. 4c). The Au 4f XPS spectrum of the
We thank the National Natural Science Foundation of China
Au/TTB-COF displays Au3+ (91.4 and 89.1 eV) and Au+ peaks (No. 51672046, 51672047, 51472050 and 21403238) and the
(87.4 and 85.1 eV). No Au0 signals could be detected before the Open Project Program of the State Key Laboratory of
reduction. After in situ reduction with Na2S, the typical XPS Photocatalysis on Energy and Environment (No. SKLPEE-
peaks of Au0 appear at 88.4 (Au 4f5/2) and 84.4 eV (Au 4f7/2) (Fig. KF201815).
4d), respectively. Meanwhile, the intensities of Au3+ and Au+ Conflicts of interest
peaks clearly decrease. The appearance of Au0 could be
assigned to the interior Au atoms in Au nanoparticles in
Au(0)/nanoparticles, while Au+ are attributed to the surface Au
atoms of the nanoparticles. Therefore, the pendant thioether
groups in TTB-COF not only provide strong Au-S interactions but
also stabilize the Au nanoparticles ions in nanoparticles. XRD
pattern for Au(0)/TTB-COF displayed characteristic peaks of
metallic Au (Fig. S29, ESI†). Au(0)/TTB-COF was then subjected
to the next Au ions capture run. Gratifyingly, the Au ions
adsorption capacity was still up to 98% even after four cycles
(Fig. 4e). Upon further pyrolysis to decompose the COF, high-
purity metallic Au is obtained (metallic Au in Fig. 4a).
There are no conflicts to declare.
Notes and references
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In conclusion, we have developed an effective COF with
pendant thioether arms for selective sensing and recovery of Au
ions in water. Significantly, given the specific coordination
interactions between thioether groups and Au ions as well as
the well-defined 2D conjugated COF structures, TTB-COF
exhibits superb fluorescence sensing and capture efficiency in
ultra-low concentration Au ions detection and recovery.
Metallic Au powder with high purity can be obtained by further
reduction and pyrolysis treatments. This work not only
highlights the feasibility of sulfur-functionalized COFs for
sensing and recovery of low concentration of Au ions in water
but also stimulates further studies toward developing more
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Ultra-low concentration of Au in aqueous solution is selectively detected and recovered by using a
covalent organic framework bearing thioether pendant arms.