Reaction-based fluorescent silk probes with high sensitivity and selectivity to Hg2+and Ag+ions

Article abstract of DOI:10.1039/d0tc05429a

The detection and removal of heavy metals in the environment is urgent and meaningful. In this work, two types of fluorescent functionalized silksOSPandASPhave been prepared using worm silk as the substrate. These fluorescent silk probes exhibit an obvious fluorescence quenching effect in the presence of Hg²⁺or Ag⁺, enabling silk to specifically detect Hg²⁺or Ag⁺. An obvious color response occurs under visible light, from yellow to brown or gray, thereby realizing dual-channel identification of fluorescence and colorimetry. In addition, its sensing mechanism has been studied, and it is found that the probe reacts with metal ions as a reactive response. Compared with the fluorescent probes bearing one C-C/C-C bond, the probes with two terminal C-C/C-C bonds are more sensitive. And the excellent recognition ability can make the limit of detection as low as 0.25 μM. This indicates that silk fluorescent probes can be used to detect Hg²⁺and/or Ag⁺

Full text of DOI:10.1039/d0tc05429a

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Materials Chemistry C
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Reaction-based fluorescent silk probes with high
sensitivity and selectivity to Hg2+ and Ag+ ions†
Cite this: J. Mater. Chem. C, 2021,
9, 4877
Li Xiao,a Kaiqi Liu,a Lian Duan*b and Xinjian Cheng
*
a
The detection and removal of heavy metals in the environment is urgent and meaningful. In this work,
two types of fluorescent functionalized silks OSP and ASP have been prepared using worm silk as the
substrate. These fluorescent silk probes exhibit an obvious fluorescence quenching effect in the
presence of Hg2+ or Ag+, enabling silk to specifically detect Hg2+ or Ag+. An obvious color response
occurs under visible light, from yellow to brown or gray, thereby realizing dual-channel identification of
fluorescence and colorimetry. In addition, its sensing mechanism has been studied, and it is found that
the probe reacts with metal ions as a reactive response. Compared with the fluorescent probes bearing
one CQC/CRC bond, the probes with two terminal CQC/CRC bonds are more sensitive. And the
excellent recognition ability can make the limit of detection as low as 0.25 mM. This indicates that silk
fluorescent probes can be used to detect Hg2+ and/or Ag+.
Received 19th November 2020,
Accepted 15th March 2021
DOI: 10.1039/d0tc05429a
rsc.li/materials-c
In order to enlarge the application scope,7 it is necessary to
modify it. Common methods of silk modification are mainly
1. Introduction
Worm silk is a natural macromolecular material composed of divided into physical modification, chemical modification,
silk fibroin, sericin, pigments, carbohydrates and inorganic genetic modification and feeding silkworm breeding.8
components.1 There are four main types of elements, C, H, O The most effective one is chemical modification. The side chain
and N in worm silk. Sericin is a water-soluble protein that coats of silk protein contains reactive groups such as hydroxyl, amino
the outside of the silk fibroin,2 and silk fibroin is a tough and and carboxyl groups, which are the basis for chemical
elastic protein. There are crystalline and non-crystalline regions modification of silk fibers.9 Vepari et al. have used cyanuric
in silk macromolecules. The crystalline region has a dense chloride-activated polyethylene glycol to chemically modify
structure and isn’t prone to chemical reactions. There are more silk fibroin, so silk fibroin can be used in anti-adhesion and
active groups on the side chain of the non-crystalline areas, and anti-blood acid.10 Teramoto et al. have used 4-cyanoisocyanate
the chemical reaction mainly takes place in this part.3 Since it to chemically modify sericin.11 And the solubility, moisture
was discovered that silk fibroin has unique chemical properties absorption and thermal stability are all significantly improved.
and good compatibility with the human body, its application Sericin solubility, hygroscopicity and thermal stability are all
range has covered many fields such as food, medicine, fine significantly improved. Various new functional materials have
chemicals, and biotechnology.4 In recent years, silk fibroin has been developed through chemical or physical processing
been discovered to have greater potential usage, such as methods,12,13 which broaden the new uses of silk.
applications in biochemical sensing materials, soft tissue
compatible materials, environmental protection and other human probes have become the latest in ‘‘silk technology’’.14
fields.5
Recently, researchers reported that they will soon be able to use
Smart socks, stage clothing, sports equipment and wearable
Silk fabric is soft and smooth and drapes well, but easily mulberry silk to develop more sensitive and flexible multi-
turns yellow and has poor elasticity.6 It is prone to get wrinkled functional equipment. It is used to monitor a large number
during wearing and washing and its shortcomings of poor of human functions in real time.15 Mulberry silk flexible strain
antibacterial and antioxidant activity limit its application. probes have broad prospects and are ideal materials for man-
ufacturing wearable human probes. They will be used as an
integrated wireless system to make it easier for doctors to
a School of Chemistry and Environmental Engineering, Wuhan Institute of
remotely monitor patients.16
Technology, Wuhan, 430073, China. E-mail: chxj606@163.com
b School of Textiles and Garments, Southwest University, Chongqing, 400715
Generally, silk is usually used as the basic material of
clothing. If fluorescent sensing groups are introduced into
P. R. China. E-mail: duan19850420@163.com
the molecular chain, their use range could be expanded.
The metal ions widely existing in nature play an important role
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0tc05429a
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in the balance of the environment and biological processes, fluorescent silk retains the detection ability of the corres-
and the excessive amount of metal ions is highly toxic to ponding grafted monomer. It is found that probes with a large
organisms.17 Excessive Hg2+ will seriously damage the central number of alkene and acetylenic bonds have higher recognition
nervous system and cause various cognitive and sports sensitivity for Hg2+ or Ag+. When Hg2+ or Ag+ is added, the
injuries.18 Ag+ can inactivate sulfur-containing enzymes in the fluorescence intensity of the probe is significantly quenched,
body, and can also combine with various metabolites of organisms and the color changes are visible to the naked eye. The alkene
to cause various diseases.19 Therefore, it is necessary to develop a and acetylene bond structures can undergo a hydroxy-mercury
simple and convenient method for detecting Hg2+ and Ag+. reaction37 in the presence of Hg2+. Meanwhile, the acetylene
The fluorescence analysis has received widespread attention with bond structure in the probe can generate metal acetylides in the
its obvious advantages among the reported ion detection methods. presence of Ag+. The fluorescent silk probe retains the detection
Boron dipyrromethene (BODIPY) has good photophysical and ability of the corresponding grafted monomer, and all
chemical properties,20 high extinction coefficient and fluorescence chemically react with Hg2+ or Ag+. Moreover, even in the
quantum yield, and good tolerance to polarity and pH.21,22 It has presence of interference from other metal ions, the probe can
been widely used in fluorescent probes, pharmacological analysis still detect Hg2+ or Ag+.
and other fields. A large number of fluorescent probes for the
detection of Hg2+ ions has been reported.23–26 However, many
probes still have shortcomings such as poor selectivity, poor
2. Experimental
2.1. Materials
biocompatibility, and difficult removal.27 Natural macromolecular
materials, especially silk, are a kind of matrix material with good
biocompatibility and excellent biodegradability. Fluorescent small
molecules could be introduced as a structural element into a
Silk, 3-bromopropyne, 2,4-dimethylpyrrole, protocatechualdehyde,
allyl bromide, p-hydroxybenzaldehyde, trifluoroacetic acid (TFA),
boron trifluoride diethyl etherate (BF3ÁEt2O) and 2,3-dichloro-5,6-
macromolecule. They can be on the main chain, on the side chain,
or in natural macromolecular materials.28–30 Using silk as a
dicyano-1,4-benzoquinone (DDQ) were purchased from J&K China
Chemical Ltd. Methanol, potassium carbonate (K2CO3), sodium
substrate, fluorescent small molecules are introduced into the
bicarbonate (NaHCO3), sodium carbonate (Na2CO3), triethylamine
(TEA), tetrahydrofuran (THF) and N,N-dimethylformamide (DMF)
skeleton, so that silk becomes a multifunctional luminescent
material, namely fluorescent silk. Compared with the initial silk,
were bought from Aladdin Chemical Co., Ltd. Dichloromethane
these functionalized silks have good fluorescence and sensing
(DCM), ethanol (EtOH), ethyl acetate (EA), petroleum ether (PE)
properties.
and acetone were bought from Sinopharm Chemical Reagent Co.,
BODIPY is an electron-rich compound with multiple positions
Ltd. Acetone, DMF, TEA and THF were purified by distillation for
on its parent moiety that can be modified by functional groups. In
order to detect Hg2+ and Ag+, specific fluorescent recognition
further use.
All metal salts (Hg2+, Ag+, K+, Ni2+, Co2+, Pd2+, Mn2+, Zn2+,
groups should be introduced on BODIPY. Special recognition
Pb2+, Cu2+, Na+, Ca2+, Fe2+, Ba2+, Mg2+, Cd2+, Au3+ and Fe3+)
were purchased from Aladdin Chemical Co., Ltd and used as
groups with metal ions can cause fluorescence and ultraviolet
absorption changes, so as to achieve detection.31 Lee et al.
received. Bovine serum albumin was purchased from Biosharp.
designed and synthesized a novel probe with BIDIPY as the
Ultrapure water (417 MO cmÀ1) from a Milli-Q water system
fluorescent group and thiosemicarbazide as the complex
recognition group.32 And it is used to simultaneously detect and
was used throughout the experiments.
remove Hg2+ in pure aqueous solutions. Das et al. synthesized a
near-infrared fluorescent probe to detect Hg2+ ions in physio-
2.2. Instrumentation
logical environments.33 The intramolecular charge transfer (ICT) The element diagram and band diagram were obtained by XPS
process of the probe is blocked after Hg2+ is complexed with the (PHI-5300; ESCA). Fourier transform infrared spectra (FTIR,
nitrogen atom in the probe. Although these probes have good 4000–400 cmÀ1) were performed on a Nicolet NEXUS 470
detection results, they are still susceptible to anions and spectrometer. 1H NMR spectra were recorded on an AVANCE
solvents.34 The detection mechanism of these probes is mainly III 400 MHz (Bruker) instrument (in CDCl3, d6-DMSO).
through complexation with metal ions to achieve recognition Mass spectrometry (LC-MS) was carried out on a Varian Pro
response. Zhou et al. synthesized an ethyleneoxy-modified MALDI instrument. The microscopic morphologies were
coumarin into a fluorescent probe for Hg2+ detection.35 Due to obtained by scanning electron microscopy (SEM, JSM-5510LV,
the specificity of the reaction, the probe showed excellent Japan). Fluorescence analysis (FL) was performed on
a
selectivity and high sensitivity, and its detection limit reached Shimadzu F-4600 spectrophotometer. UV-Vis absorption spec-
0.12 mM. At present, there are relatively few Ag+ fluorescent probes tra were obtained by UV-5900PC spectrophotometer (METASH).
based on chemical reactions.
TGA was analyzed by thermogravimetric analysis (TGA, 209F3
In this work, BODIPY with one or two alkene and acetylene Tarsus [NETZSCH] instrument, Frankfurt, Germany). Differen-
bonds is used as the fluorescent precursor. Then two C–I bonds tial scanning calorimetry (DSC) was tested on a NET ZSCH DSC
are introduced at the 2,6 positions of BODIPY via halogenation 200F3240-200801-L instrument. The adsorption of Hg2+ on silks
reaction. Subsequently, BODIPY monomer probes were grafted was tested by atomic absorption spectrometry (Varian SpetrAA
onto silk through substitution reactions,36 exploring whether 220FS).
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2.3. Synthesis
2.3.2. Synthesis of precursors of fluorescent silk (BI1, BI2,
BI3, BI4). BODIPY derivatives were prepared using the reported
method.39 A1/A2/A3/A4 (1 mmol) and 2,4-dimethylpyrrole
(2.25 mmol) was dissolved in THF (50 mL) under the protection
of N2, 30 mL of TFA was slowly dropped into (26 1C for 6 h). DDQ
(2 mmol) was added to the reaction solution at 0 1C. After
stirring for 3 h, TEA (3.5 mL) and BF3ÁEt2O (3.7 mL) were
continued to be added to the reaction system at 0 1C, and
reacted overnight. The mixture was purified by column
chromatography to obtain the fluorescent solid product.
Using 1 mmol BODIPY derivative (BS1, BS2, BS3, BS4) as the
starting material and 20 mL DCM as the solvent, the iodination
reaction was quickly completed with 8 mmol NIS within 1 h.
Then, it was extracted and purified by column chromatography
using PE/EA as the eluent to obtain a red powder.
The synthetic route of fluorescent silk is shown in Scheme 1,
and the synthetic route of precursor small molecules is shown
in ESI.†
2.3.1. Synthesis of 4 or 3,4(ene/propargyloxy) benzaldehyde
(A1, A2, A3, A4). A1/A2/A3/A4 was synthesized according to
modified literature procedures.38 The starting material
was dissolved in acetone and an excess of K2CO3 was added. The
mixture was reacted at 50 1C for 6 h. After that the mixture was
concentrated and purified via silica gel column chromatography.
4-Mono(allyloxy)benzaldehyde (A1). TLC (PE/EA = 22 : 1, v/v).
Oil, yield (82%). 1H NMR (400 MHz, CDCl3, ppm): d 9.88 (s, 1H),
7.83 (d, 2H), 7.01 (d, 2H), 6.05 (d, 1H), 5.43 (d, 1H), 5.36–5.28
(m, 1H), 4.62 (d, 2H). 4-Mono(prop-2-yn-1-yloxy)benzaldehyde
(A2). TLC (PE/EA = 15 : 1, v/v). White solid, yield (76%). 1H NMR
(400 MHz, CDCl3, ppm): d 9.90 (s, 1H), 7.86 (d, 2H), 7.09 (d, 2H),
4.78 (d, 2H), 2.57 (t, 1H). 3,4-Bis(allyloxy)benzaldehyde (A3).
4,4-Difluoro-8-[4-allyloxy]-1,3,5,7-tetramethyl-4-bora-3a,4a-
diaza-s-indacene (BS1). TLC (PE/EA = 36 : 1, v/v). Orange red
1
1
solid, yield (51%). H NMR (400 MHz, CDCl3, ppm): d 7.16 (d,
TLC (PE/EA = 30 : 1, v/v). Oil, yield (85%). H NMR (400 MHz,
2H), 7.02 (d, 2H), 6.14–6.04 (m, 1H), 5.97 (s, 2H), 5.45 (d, 1H),
5.33 (d, 1H), 4.59 (d, 2H), 2.54 (s, 6H), 1.42 (s, 6H). ESI-HRMS
DMSO, ppm): d 9.80 (s, 1H), 7.49 (d, 1H), 7.38 (d, 1H), 7.11
(d, 1H), 6.12–5.90 (m, 2H), 5.41 (d, 2H), 5.30–5.15 (m, 2H), 4.63
(d, 4H). 3,4-Bis(prop-2-yn-1-yloxy) benzaldehyde (A4). TLC (PE/ (m/z): [M + H]+ calcd for C22H23BF2N2O: 381.19, found 381.1949.
1
EA = 22 : 1, v/v). White solid, yield (78%). H NMR (400 MHz, 4,4-Difluoro-8-[4-prop-2-yn-1-yloxy]-1,3,5,7-tetramethyl-4-bora-3a,
CDCl3, ppm): d 9.88 (s, 1H), 7.60–7.54 (m, 1H), 7.26 (s, 1H), 7.18 4a-diaza-s-indacene (BS2). TLC (PE/EA = 30: 1, v/v). Pink solid,
yield (55%). 1H NMR (400 MHz, CDCl3, ppm): d 7.25–7.16
(d, 1H), 4.85 (d, 4H), 2.57 (d, 2H).
Scheme 1 Synthetic route of fluorescent silk (OSP and ASP).
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(m, 2H), 7.09 (t, 2H), 5.98 (s, 2H), 4.76 (s, 2H), 2.58–2.50 (m, 6H), solution was added and shaken well. The silk fabric sample
2.43–2.16 (m, 1H), 1.42 (s, 6H). ESI-HRMS (m/z): [M + H]+ calcd was put into a three-necked flask and sealed, and then placed
for C25H23BF2N2O2: 378.16, found 378.1822. 4,4-Difluoro-8-[3,4- in a constant temperature stirring water bath to shake for 1 h.
diallyloxy]-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene (BS3). Subsequently, the temperature was increased to 80 1C for 24 h.
TLC (PE/EA = 40 : 1, v/v). Orange red solid, yield (48.5%). The reacted silk fabric sample was thoroughly washed with
1H NMR (400 MHz, DMSO, ppm): d 7.11 (d, 1H), 6.93 (d, 1H), water and methanol, and dried in a vacuum drying oven.
6.81 (d, 1H), 6.15 (s, 2H), 6.05–5.93 (m, 2H), 5.44–5.32 (m, 2H),
5.22 (d, 2H), 4.59 (d, 4H), 2.36 (d, 6H), 1.31 (d, 6H). ESI-HRMS
(m/z): [M + H]+ calcd for C25H27BF2N2O2: 437.21, found 437.2210.
4,4-Difluoro-8-[3,4-bisprop-2-yn-1-yloxy]-1,3,5,7-tetramethyl-4-bora-3a,4a-
3. Results and discussion
3.1. Structure characterization
diaza-s-indacene (BS4). TLC (PE/EA = 25 : 1, v/v). Bright red
solid, yield (52%). 1H NMR (400 MHz, CDCl3, ppm): d 7.16 First, CQC/CRC is introduced onto BODIPY small molecules,
(d, 1H), 6.98 (d, 1H), 6.89 (d, 1H), 5.98 (s, 2H), 4.80 (d, 4H), and fluorescent small molecule probes with different
2.59–2.50 (m, 6H), 2.43 (t, 2H), 1.48 (s, 6H). ESI-HRMS (m/z): recognition sites were prepared. Then, two C–I bonds are
[M + H]+ calcd for C25H23BF2N2O2: 433.18, found 433.1898.
introduced at positions 2,6 of BODIPY via a halogenation
Precursors of fluorescent silk (BI1). TLC (PE/EA = 60 : 1, v/v). reaction to obtain fluorescent silk precursors BI3 and BI4.
1
Red powder, yield (89%). H NMR (400 MHz, DMSO, ppm): d Finally, BODIPY grafted fluorescent silk probes OSP and ASP
7.28 (d, 2H), 7.13 (d, 2H), 6.11–6.00 (m, 1H), 5.42 (d, 1H), 5.28 are obtained through a substitution reaction.
(d, 1H), 4.63 (d, 2H), 2.52 (s, 6H), 1.38 (s, 6H). ESI-HRMS (m/z):
3.1.1. FT-IR analysis. The infrared spectra of the fluorescent
[M]+ calcd for C22H21BF2I2N2O: 631.98, found 631.96. Precur- silk grafted with BODIPY derivatives of different functional
sors of fluorescent silk (BI2). TLC (PE/EA = 100 : 1, v/v). Red groups are shown in Fig. 1a.
1
powder, yield (85%). H NMR (400 MHz, DMSO, ppm): d 7.31
S0 is un-degummed silk and P0 is degummed silk. The
(d, 2H), 7.16 (d, 2H), 4.89 (s, 2H), 3.61 (s, 1H), 2.52 (s, 6H), 1.37 characteristic absorption peaks of silk fibroin can be seen in the
(s, 6H). Precursors of fluorescent silk (BI3). TLC (PE/EA = 75 : 1, infrared spectra (d, c) of S0 and P0, such as amide I (1628 cmÀ1),
v/v). Red powder, yield (88%). 1H NMR (400 MHz, DMSO, ppm): amide II (1520 cmÀ1), and amide III (1233 cmÀ1).40 However,
d 7.14 (d, 1H), 6.98 (s, 1H), 6.85 (d, 1H), 6.03 (s, 2H), 5.30 these characteristic peaks were not seen in the infrared spectra
(d, 4H), 4.60 (d, 4H), 2.52 (s, 6H), 1.42 (s, 6H). ESI-HRMS (m/z): of grafted silk. It shows that the degummed silk still has a silk
[M]+ calcd for C25H25BF2I2N2O2: 688.01, found 688.07. fibroin b-sheet structure, while the grafted silk does not have this
Precursors of fluorescent silk (BI4). TLC (PE/EA = 70 : 1, v/v). structure. The infrared spectrum of the grafted silk OSP showed
1
Red powder, yield (82%). H NMR (400 MHz, DMSO, ppm): d new peaks at 1634 cmÀ1 and 3054 cmÀ1, which were CQC and
7.20 (s, 1H), 7.08 (s, 1H), 6.94 (d, 1H), 4.88 (s, 4H), 3.58 (t, 2H), QC–H stretching vibration peaks respectively. And the infrared
2.52 (s, 6H), 1.44 (s, 6H).
spectrum of ASP shows a new peak at 2186 cmÀ1, which is the
2.3.3. Synthesis of fluorescent silk (OSP, ASP). 6 g LÀ1 characteristic peak of CRC bond. However, these characteristic
sodium carbonate solution was configured to degumming peaks do not appear in the infrared spectrum of degummed silk,
pretreatment for silk fabric. The sample was fully immersed so it can be proved that the fluorescent monomer has been
in a boiling pot. It was taken out, washed, and dried for later successfully grafted to the surface of the silk fabric. The infrared
use after being degummed at 100 1C for 1 h.
spectra of small molecules are shown in Fig. 1b and Fig. S4
1 g of the degummed silk fabric was washed with DMF and (ESI†).
dried for 10 h in a vacuum at 40 1C. Then it was placed in a
3.1.2. 1H NMR analysis. Fig. S1–S3 (ESI†) show the 1H NMR
desiccator to be cooled for 30 min. 50 mg of fluorescent spectra of each small molecule. As shown in Fig. S2a (ESI†), the
precursors (BI3, BI4) was put into a three-necked flask and aldehyde hydrogen and terminal alkyne active hydrogen in the
dissolved in 50 mL DMF. An excess of potassium carbon A2 structure appear at 9.90 ppm and 2.58 ppm, respectively.
Fig. 1 FTIR spectrum of (a) ASP, OSP, P0, S0, and (b) A4, BS4, BI3, BI4.
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And the aldehyde hydrogen was not found in the BS2 spectrum image that the surface of the silk becomes rougher, but the
after the reaction, indicating that A2 has been completely distribution of particulate matter on the surface is still rela-
consumed (Fig. S2b, ESI†). The characteristic peak position tively uniform. This layer of granular material almost comple-
disappears at 6.02 ppm in Fig. S2c and d (ESI†), indicating that tely covers the surface of the fiber. This layer of granular
the active hydrogens at positions 2,6 of the BODIPY parent material can be treated with hot water in the presence of
structure in BS2 and BS4 have been replaced. Similarly, Fig. S1 surfactants, so it relies on the chemical bond to firmly bond
and S3 (ESI†) prove that other small molecules have been with the silk fiber. This may be because the grafted fluorescent
successfully synthesized. Therefore, the position and number monomer destroys the N–H and O–H bonds in the molecule,
of hydrogens in each molecular structure are consistent with which causes the surface of the silk fiber to become uneven.
the experimental design.
In addition, the particulate matter is a grafted fluorescent
3.1.3. XPS analysis. XPS analysis is performed to further monomer. The modification makes silk
a
promising
confirm the changes in the silk structure. As shown in Table 1 luminescent material.
and Table S1 (ESI†), the original silk contains three elements: N
3.2.2. Thermal performance analysis. The thermal properties
(399 eV), O (532 eV) and C (285 eV). The introduction of of silk before and after degumming and grafting are checked by
fluorescent small molecules (BS3 and BS4) with different thermogravimetric analysis (TGA) and differential scanning
structures, resulting in different degrees of changes in the calorimetry (DSC). The decomposition of degummed silk (S0, P0)
atomic content of each element in the grafted silk. Moreover, is divided into two stages (Fig. S12, ESI†). The silk starts to
the XPS spectra of OSP and ASP show three new element peaks become dehydrated at about 100 1C and decomposes at 305 1C.
B (192 eV), F (685 eV) and I (619 eV). It shows that BODIPY The total mass loss of grafted silk ASP is 57% (the main mass loss
derivatives have been successfully introduced into the silk is about 310 1C). After heating the silk polymer to 600 1C under
structure through some active N–H and O–H bonds. It is also nitrogen, the grafted silk may leave more residual material (up to
found that element I still remains in the fluorescent silk. After 43%) due to the formation of BODIPY with a strong rigid
BI3/BI4 reacts with the worm silk mat, both the I atoms did not structure. These data support the suggestion that the grafting
react. This may be the result of reaction probability. If both monomer has an effect on the thermal properties of silk.
ends react, the small molecules should bend, which is difficult
Fig. S13 (ESI†) is the DSC curve of modified silk grafted with
to achieve due to steric hindrance. However, only one end different BODIPY monomers. The degummed silks S0 and P0
reacts, leaving the I at one end, which is easy to achieve. Based have obvious endothermic peaks near 322 1C and 330 1C,
on the above analysis results, it can be determined that the respectively, which are caused by the thermal decomposition
fluorescent silk has been successfully prepared.
of silk fibroin with the b-sheet structure.41 However, the thermal
Thence, the structure of each fluorescent probe is completely decomposition endothermic peaks of grafted silk OSP (344 1C)
consistent with the design combining FT-IR, NMR, MS and XPS and ASP (378 1C) tend to move upward, indicating that the
analyses.
thermal stability of the silk is improved after monomer grafting.
And the absorption peak area of the grafted silk becomes
smaller, that is, the enthalpy decreases as the grafting pro-
gresses. This may be owing to the decrease in the relative content
of silk protein in the grafted polymer, and the graft chain acts as
a ‘‘diluent’’ for silk protein.42 A new endothermic peak appears
at 580–600 1C, which is attributed to the specific thermal
transition after the silk is grafted with BODIPY with strong rigid
structure. At the same time, it further proves that the BODIPY
monomer has successfully grafted the surface of silk.
3.2. Characterization of physical properties of fluorescent silk
3.2.1. SEM analysis. Fig. S11 (ESI†) shows the longitudinal
morphology of silk fiber. The silk surface without any treatment
is smooth. The surface of the silk fiber becomes rough after
degumming (Fig. S11b, ESI†), which changes the original
smooth appearance of the silk fiber. It increases the irregular
reflection of light on the fiber surface and increases the
absorption rate, thus having a visual texture enhancement
effect. However, a lot of particulate matter is formed on the
silk fiber grafted with BODIPY fluorescent monomer (Fig. S11c
and d, ESI†). It is not difficult to find through the enlarged
3.3. Detection of heavy metal ions
3.3.1. Recognition performance of BS1, BS2 for metal ions.
The recognition performance of BS1 and BS2 for 18 metal
ions is shown in the ultraviolet absorption and fluorescence
emission spectra of Fig. 2. All tests are performed in a
DMF/buffer system (8 : 2, rt, pH = 7.0). It can be seen from
Fig. 2(a and c) that the UV-Vis absorption and fluorescence
emission peak of probe BS1 appear at 500 nm and 530 nm,
respectively. BS1 only responds to Hg2+. When Hg2+ is added to
BS1, the UV-visible absorption peak of BS1 is red-shifted from
500 nm to 511 nm. In addition, the intensity of the fluorescence
peak is weakened with a red shift of 10 nm wavelength, which is
manifested as fluorescence quenching.
Table 1 The content of elemental components in untreated silk (S0, P0)
and fluorescent silk (OSP, ASP) was obtained by XPS analysis
Atomic %
Elements
S0
P0
OSP
ASP
C
N
O
B
F
I
71.40
7.79
20.81
—
—
—
68.81
9.98
21.22
—
—
—
69.75
3.41
23.72
0.83
1.51
0.78
66.32
4.81
24.28
1.23
2.32
1.04
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Fig. 2 UV absorption spectra of small molecule probes (10 mM) in the presence of 10 equivalents of metal ions in DMF-buffer (pH = 7.0, rt) (a) BS1, (b)
BS2. Fluorescence emission spectra of small molecule probes (10 mM) in the presence of 10 equivalents of metal ions in DMF-buffer (pH = 7.0, rt) (c) BS1,
(d) BS2.
Both the UV-Vis absorption and fluorescence emission shows the changes in the fluorescence intensity of BS2 at
intensity of BS2 change after adding Hg2+ or Ag+, and the different concentrations of Ag+. When the concentration of
fluorescence is significantly quenched up to 90%. There is no Ag+ is 0.10 mM, the intensity of BS2 starts to decrease. The
obvious shift (410 nm) in the fluorescence emission wavelength, fluorescence intensity decreases with the Ag+ concentration
but the fluorescence intensity is significantly quenched. This increases, and finally reaches saturation. The fluorescence
fluorescence change can be explained by photoelectron transfer quenching rate can be as high as 88%, which indicates that
theory (PET).43 BS2 shows significant fluorescence quenching BS2 is highly sensitive to the recognition of Ag+. This may be
caused by Hg2+ or Ag+; however, BS1 only shows weak fluores- due to the terminal acetylene bond in the BS2 coordinated with
cence quenching caused by Hg2+. This difference occurs because Ag+ to form heavy metal acetylene for specific recognition.
the acidity of CRC–H is stronger than CQC–H. BS2 combine Analogously, the titration response of BS2 and Hg2+ is similar.
with Hg2+ or Ag+ ions to form salt easily.44 The hydrogen acidity of
Fig. 3d–f is the linear relationship between fluorescence
the terminal ethylenic bond in BS1 is weaker than that of BS2, and intensity and concentration of Hg2+/Ag+. It is not difficult to
it can only coordinate with Hg2+. Therefore, the recognition effect see that the fluorescence intensity of these probes gradually
of BS2 is more sensitive and should stimulate a significant decreases with the increase of ion concentration, showing a
fluorescence quenching effect.
good linear relationship (0.9875, 0.9944 and 0.9964). Use the
It can be seen from Fig. S16 (ESI†) that only Hg2+ can cause a following formula to obtain the limit of detection45 (LOD):
significant change in the color of BS1 under natural and
UV light. After the reaction, the color of the solution changes
from light yellow to orange–red, and the bright green fluorescence
is quenched. Therefore, the colorimetric/fluorescence naked eye
LOD = 3s/K.
Here, s is the standard deviation of the blank probe
detection of Hg2+ can be realized. Additionally, only Hg2+/Ag+ can solution, and K is the slope of the standard curve. The LOD is
cause a significant change in the color of BS2, while other metal listed in Table 2. And it can be found that BS1, BS2, BS3 and
ions retain the original color of the probe. All the above results BS4 all have good sensing capabilities for Hg2+. BS2 and BS4
indicate that these fluorescent probes can selectively detect Hg2+ also have good sensitivity to Ag+. It proves that they are ideal
or Ag+.
probes for detecting Hg2+ or Ag+. Additionally, it can be found
3.3.2. Sensitivity of small molecule fluorescent probes to that BS3/BS4 are more sensitive to metal ions than BS1/BS2 by
Hg2+/Ag+. In order to further understand the recognition effect comparison. This may be because BS1/BS2 contains one CQC/
of the probe on Hg2+/Ag+, a fluorescence titration experiment CRC structure, while BS3/BS4 have two CQC/CRC structures.
was carried out. The fluorescence intensity of BS1 (Fig. 3a) BS3 and BS4 have more reaction sites with metal ions, which
decreases with the increase of Hg2+ solution concentration, and increases the chance of binding and improves the recognition
the position of the fluorescence peak also red-shifts. Fig. 3c sensitivity. In addition, some Hg2+ and Ag+ probes46–49 have been
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Fig. 3 (a) Concentration effect of BS1 on Hg2+. (b) Concentration effect of BS2 on Hg2+. (c) Concentration effect of BS2 on Ag+. (d) Concentration-
dependent fluorescence signaling of a Hg2+ by BS1, (e) Hg2+ by BS2, and (f) Ag+ by BS2.
supplemented to ESI† and their detection performance has been CQC bond, while BS3 has two terminal CQC bonds. Similarly,
compared. By comparing the LOD, it is found that the reactive the stoichiometric ratio of BS2 and Hg2+/Ag+ is 1 : 1, and the
fluorescent probe in this work has more sensitive detection stoichiometric ratio of BS4 and Hg2+/Ag+ is 1 : 2. This is because
performance for Ag+ ions.
BS2 has only one terminal CRC bond, while BS4 has two
3.3.3. Job’s plot analysis for fluorescent probes. In order to terminal CRC bond. It is consistent with the results of the
understand the recognition performance more clearly, Job’s aforementioned fluorescence titration experiment, and also
analysis is used to calculate the stoichiometric ratio of each supports the mechanism speculation in Scheme S2 (ESI†).
probe to metal ions. The remaining corresponding studies of
3.3.4. Interference of other ions and albumin to the
BS3 and BS4 have been completed in our previous work. The fluorescent probes. Fig. S15a–c (ESI†) shows that the presence
specific test is as follows: keep the total concentration of probe of other metal ions and bovine serum albumin has no significant
and ion at 10 mM, and constantly change the probe to ion ratio. effect on the probe’s specific recognition of Hg2+ or Ag+. It
The ratio of ion concentration to total concentration is taken indicates that only in the presence of Hg2+, the alkene bond of
as the abscissa. The difference between the actual value (A) and the probe can be changed. And the acetylene bond structure of
the blank value (A0) is the ordinate.50 The inflection point of the the probe can be changed in the presence of Hg2+/Ag+, while the
curve is the stoichiometric ratio between the probe and the conjugate structure is not affected in the presence of other metal
metal ion.
The results are shown in Fig. 4(a–f), and the stoichiometric
ions. Therefore, these probes can be used in most environments.
3.3.5. Response performance of fluorescent silk probes to
ratio is shown in Table 2. It can be seen from the analysis of the Hg2+ or Ag+. Since BS3/BS4 are more sensitive than BS1/BS2, the
results that the stoichiometric ratio between BS1 and Hg2+ is fluorescent silk probes OSP/ASP containing two CQC/CRC
1 : 1, and the stoichiometric ratio between BS3 and Hg2+ is 1 : 2. bond structures is selected for the metal ion response test. It
This may be due to the fact that BS1 contains only one terminal can be seen from Fig. 5b that the fluorescence emission peak of
OSP appears at 420 nm. Other metal ions have not yet caused
changes in the fluorescence emission peaks. However, Hg2+
Table 2 The UV-Vis absorption, excitation, emission and LOD of probes causes a significant decrease in the fluorescence intensity of
(BS1, BS2, BS3, BS4, OSP, ASP)
OSP, which is manifested as fluorescence quenching. The UV
absorption peak of OSP (Fig. 5a) appears at 450 nm. And the UV
labs
(nm)
lem lex
(nm) (nm)
LOD
(mM)
Stoichiometric
ratio
absorption peak of OSP changed significantly after adding
Hg2+. Thus, the dual-channel specific detection of Hg2+ by
OSP can be realized.
Unlike OSP, ASP also undergoes significant fluorescence
quenching under the action of Ag+. When Ag+ is added, the
UV absorption peak of ASP red-shifted from 450 nm to 501 nm.
Also, a particularly obvious color change can be seen in the
actual picture. It shows that ASP has an excellent detection
effect on Hg2+ and Ag+. In addition, silk introduced small
Probe
BS1
500
511
499
510
500
335 530
335 540
340 532
340 543
330 528
—
1.61
—
0.79
—
—
1 : 1
—
1 : 2
—
BS1 + Hg2+
BS3
BS3 + Hg2+
BS2
BS2 + Hg2+/BS2 + Ag+ 503/501 330 526/524 1.42/0.90 1 : 1/1 : 1
BS4
501
335 528
—
—
BS4 + Hg2+/BS4 + Ag+ 509/502 335 539/526 0.83/0.25 1 : 2/1 : 2
OSP
ASP
450
451
330 420
330 415
—
—
—
—
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Fig. 4 Job’ plot analysis of (a) BS1 with Hg2+, (b) BS2 with Hg2+, (c) BS2 with Ag+, (d) BS3 with Hg2+, (e) BS4 with Hg2+, (f) BS4 with Ag+.
Fig. 5 UV absorption spectrum of fluorescent silk in the presence of metal ions in DMF-buffer (pH = 7.0, rt). (a) OSP, (c) ASP. Fluorescence emission
spectrum of fluorescent silk in the presence of metal ions in DMF-buffer (pH = 7.0, rt). (b) OSP, (d) ASP.
molecule probes through a substitution reaction also showed P0 has the ability to adsorb Hg2+ due to –COOH can adsorb
perfect detection performance for Hg2+ or Ag+. Therefore, Hg2+ to form carboxylate; the lone pair of electrons on the N
fluorescent silk retains the detection performance of the atom of –NH2 can coordinate with Hg2+ as well. The adsorption
grafted monomer probe.
capacity of OSP and ASP is stronger because the terminal CQC/
3.3.6. The application of fluorescent silk as absorbent for CRC adsorbs Hg2+. In addition, the XPS test was also carried
Hg2+ in water. The fluorescent silk as Hg2+ indicator and out, and the Hg2+ content bound to fluorescent silk was further
absorber were further studied by atomic absorption spectro- obtained. The results are in Table S2 (ESI†). It is found that this
meter (AAS). Table S2 (ESI†) shows the adsorption capacity of value is less than the calculated adsorption efficiency. XPS tests
degummed silk P0 and fluorescent silk OSP/ASP to Hg2+. It is the adsorption of the sample surface; and the AAS tests the
found that ASP (2.05% mgÀ1) has the largest adsorption efficiency adsorption of both surface and inside the sample. Therefore,
for Hg2+, followed by OSP (1.78% mgÀ1), and P0 (1.49% mgÀ1). compared with the adsorption capacity of silk itself, the
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Fig. 6 (a) 1H NMR of BS1 and equimolar amount of Hg2+ in DMSO-d6, (b) 1H NMR of BS2 and equimolar amount of Hg2+ in DMSO-d6. (c) FTIR and (d)
XPS of fluorescent silk combined with Hg2+/Ag+.
Fig. 7 (a) Photographs of fluorescent silk ASP with heavy metal ion in DMF under natural light. (b) The series of photos of silk under natural light and UV
light, showing untreated silk, fluorescent silk, silk-Hg2+, and silk-Ag+.
adsorption effect of fluorescent silk on Hg2+ is improved to a analysis by XPS (Fig. 6d) verified that fluorescent silk recognizes
certain extent.
Hg2+ or Ag+ through chemical bonding rather than physical
bonding.
It is speculated that the recognition reaction sites of small
molecule probes should be consistent with fluorescent silk.
This conjecture is indeed confirmed by infrared analysis, and
the analysis results are shown in Fig. S14 (ESI†). Subsequently,
the 1H NMR titration experiment is carried out for further
verification. After the probes BS1 (Fig. 6a) and BS2 (Fig. 6b)
are added with the same amount of Hg2+, the chemical shift
and integral area of each hydrogen near the terminal olefinic
and acetylenic bond structure change significantly. Therefore,
BS1 and BS2 are chemically combined with Hg2+ (Fig. 7).
3.4. Detection mechanism
FTIR analysis proves the binding mechanism of fluorescent silk
with Hg2+ or Ag+. The results are shown in Fig. 6c, OSP has a
characteristic peak of the CQC bond at 1634 cmÀ1. This
characteristic peak disappears after interacting with Hg2+
and a new O–H characteristic peak appears at 3557 cmÀ1
,
.
It shows that the terminal olefinic structure of the probe
molecule has changed.
Similarly, the CRC bond stretching vibration peak at
2186 cmÀ1 disappears after ASP interacts with Ag+, indicating
that the terminal acetylene bond has changed, and silver
acetylide may have been generated. Because the terminal
alkyne hydrogen is easily dissociated to produce hydrogen
4. Conclusion
protons, it is acidic and easily replaced by metal Ag.51 After In this work, four novel reactive fluorescent small molecule
ASP interacts with Hg2+, the characteristic peak of CRC probes (BS1, BS2, BS3, BS4) for detecting Hg2+/Ag+ have been
disappears, while the characteristic peak of CQO appears at designed and synthesized. The fluorescent probes containing
1706 cmÀ1. This shows that the terminal acetylenic bond reacts terminal CQC/CRC bonds show high sensitivity and selectivity
with Hg2+ to become an aldehyde group. Afterwards, further to Hg2+ or Ag+. Mechanism studies have shown that the terminal
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15 F. He, X. You, H. Gong, Y. Yang, T. Bai, W. Wang, W. Guo, X. Liu
and M. Ye, ACS Appl. Mater. Interfaces, 2020, 12, 6442–6450.
16 L. Feng, W. Chen, J. Li, G. Day, H. Drake, E. Joseph and
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CQC/CRC bonds of these four probes undergo hydroxy-
mercury reaction in the presence of Hg2+, and the detection
limit of Hg2+ reaches 0.79 mM. In addition, the terminal alkyne
hydrogens of BS2 and BS4 are easily dissociated into hydrogen
protons under the action of Ag+, and metal acetylides are easily
formed. The detection limit for Ag+ can be as low as 0.25 mM.
BODIPY-silk has been successfully synthesized. These fluorescent
silks also show perfect detection of Hg2+ or Ag+. Therefore, this
work can further expand the application range of silk, and
provides an method for the synthesis of highly efficient and
sensitive Hg2+ or Ag+ reactive probes.
´
´
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Conflicts of interest
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B. Haupler, T. Janoschka, S. Morgenstern, M. D. Hager
The authors declare that they have no competing interests.
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Acknowledgements
The authors would like to thank the National Natural Science
Foundation of China (NSFC, grant no. 41573106) for the
financial support.
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