New Strategy for Functionalization of Silica Materials via Catalytic Oxa

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New Strategy for Functionalization of Silica Materials via Catalytic Oxa-Michael Reaction of Surface Silanol Groups with Vinyl Sulfones Fang Cheng, Hanqi Wang, Wei He, Bingbing Sun, Jing Zhao, Jingping Qu, and Qing Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05550 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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ACS Sustainable Chemistry & Engineering

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New Strategy for Functionalization of Silica Materials via Catalytic Oxa-

2

Michael Reaction of Surface Silanol Groups with Vinyl Sulfones

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Fang Cheng *,1,2, Hanqi Wang 1,2, Wei He 1,3, Bingbing Sun 1,4, Jing Zhao 1, Jingping Qu1, Qing

4

Wang 1,2

5

1 State

6

2

7

116024

8

3

9

116023, China.

Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, China, 116024

School of Pharmaceutical Science and Technology, Dalian University of Technology, Dalian, China,

Department of Polymer Science and Engineering, Dalian University of Technology, Dalian, Liaoning

10

4 School

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* To whom correspondence should be addressed: [email protected]

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KEYWORDS: surface functionalization; silica materials; vinyl sulfones; catalytic oxa-Michael

13

reaction

of Chemical Engineering, Dalian University of Technology, Dalian, Liaoning 116023, China.

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ACS Paragon Plus Environment 1

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ABSTRACT

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This paper demonstrated the catalytic oxa-Michael reaction of inorganic silanol groups with

3

vinyl sulfones, which facilitates an efficient strategy for functionalization of the silica surface. The

4

strategy was applied on materials ranging from nanoscale to macroscale silica, and the surface

5

functionalization was achieved in hours using organo-catalysts at mild temperature. The formation

6

of Si-O-C bonds on the surface was characterized by solid-state 13C CP-MAS NMR, FTIR and

7

XPS. Our strategy showed several advantages over traditional methods, and the resulting Si-O-C

8

bond exhibited distinct behaviors towards different solvents. Organic solvents would stabilize the

9

functionalized silica materials, while aqueous solutions would result in degradation affected by

10

both solution and surface factors. Using divinyl sulfone as a crosslinker, a variety of molecules

11

can be immobilized and sequentially released in a controllable manner, which would benefit a

12

broad range of applications from sensing to drug and catalyst carriers.

13

Introduction

14

Silica-based materials are widely used in the fields of catalysis1-2, chromatography3-4,

15

biomaterials5-6, biosensing7-8 and drug delivery9-10 due to their earth abundance and low cost.

16

Success of these applications heavily relies on the functionalization of silica surfaces, among

17

which the conjugation of organic molecules is preferred, as shown in Scheme 1a.11 One of the

18

bonds involved in such covalent binding is the Si-O-Si bond, usually formed by the reaction of

19

surface silanol groups with organosilanes. Thus, a series of silane compounds with various

20

terminal groups have been synthesized for different applications.11-16 However, silane compounds

21

are quite reactive towards hydrolysis and tend to self-polymerize. Furthermore, silane

22

functionalization often results in multilayer structures, making it difficult to control the


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functionalization density and reproducibility.17 Covalent attachment of organophosphonate is a

2

promising alternative for the modification of inorganic oxide materials including silica. This

3

method takes advantage of the organized arrangement of amphiphilic molecules at the liquid-gas

4

interface and the transfer of this arrangement to the substrate surface before annealing to secure

5

covalent bonding.18-19 However, several factors, e.g., temperature, concentration, pH, and nature

6

of the solvent, should be taken into account as dissolution and precipitation may compete with

7

surface modification.20 Covalently anchoring alcohols onto silica surface is an old method for

8

silica functionalization, facilitated by condensation reaction of alcohols with surface silanol

9

groups.21 However, high temperature is always required to form Si-O-C bond, and low

10

functionalization coverage are the disadvantages.22 Modification of silica surface through the Si-

11

H bond has also been reported to produce monolayer functionalization. The organic molecules can

12

be immobilized by the Si-H bond reaction with a variety of groups, e.g., alkyne, alkene, thiol,

13

amine, alcohol, aldehyde and ketone, under mild conditions,23-24 among which the reaction with

14

alkenes and alkynes were referred as click reaction.25 Despite its versatility, pretreatment of silica

15

materials with HF/NH4F is required to yield active Si-H surface26. Moreover, the Si-H bond is

16

sensitive to oxygen and moisture, the reaction should be conducted in water- and oxygen-free

17

conditions in order to suppress the regrowth of silicon oxides.24

18

In this paper, we reported a convenient strategy for the functionalization of silica surface with

19

Si-O-C bonds, which are formed by the catalytic oxa-Michael reaction of surface silanol groups

20

with vinyl sulfones. Specifically, silica surfaces displaying silanol groups were either dispersed or

21

immersed in the aprotic solution27 of vinyl sulfones in the presence of catalysts, and

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functionalization was achieved in a few hours at 25~60℃, as illustrated in Scheme 1b. Using silica

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nanoparticles as samples, the formation of Si-O-C bond was characterized using dynamic light



Page 4 of 34

1

scattering (DLS) measurements, solid-state 13C cross polarization - magic angle spinning nuclear

2

magnetic resonance spectroscopy (solid-state

3

spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Furthermore, the effects of

4

solvents and pH on the stability of resulting Si-O-C bond were investigated. Using divinyl sulfone

5

(DVS) as a crosslinker, silica nanoparticles were further fluorescently functionalized by attaching

6

Cy3, and the controlled release of fluorescence molecule was examined by tuning the pH values.

13C

CP-MAS NMR), Fourier transform infrared

7 8

Scheme 1. Functionalization of silica surface via (a) traditional strategies and (b) the catalytic oxa-

9

Michael reaction of surface silanol groups with vinyl sulfones.

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Experimental

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Materials and Equipment


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Stöber silica (AS-40TM, a diameter of 20 nm, a surface area of 135 m2/g), fumed silica (a

2

diameter of 0.1~0.2 μm, a surface area of 200 m2/g) and mesoporous silica (MCM 41, a pore

3

diameter of 4 nm, a surface area of 1000 m2/g) were purchased from Sigma-Aldrich (St. Louis,

4

MO, USA). N, N’-dimethylpyridine (DMAP, 99 %), triethylenediamine (DABCO, 98 %), 1-

5

methylimidazole (1-MIM, 98 %), triphenylphosphine (PPh3, 99 %), triisopropyl-phosphine (TIPP,

6

98 %), tricyclohexylphosphine (TCHP, 96 %) and phenyl vinyl sulfone (PVS, 98%) were

7

purchased from Aladdin Industries Co. Ltd. (Shanghai, China). Divinyl sulfone was acquired from

8

Xiya Reagent (Shandong, China). 1-(6-((6-ammoniohexyl)amino)-6-oxohexyl)-3,3-dimethyl-2-

9

((E)-3-((E)-1,3,3-trimethyl-5-sulfonatoindolin-2-ylidene)prop-1-en-1-yl)-3H-indol-1-ium-5-

10

sulfonate (Cy3, a water-soluble analog of cyanine 3 terminated with amino group, the structure is

11

illustrated in Figure S1) was purchased from Ruixi Biological Technology (Xi’an, China). Vinyl

12

sulfone group terminated polyethylene glycol 2000 (PEG 2000-VS) was synthesized according to

13

our previous report.27

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Solid-state 13C

13C

cross polarization - magic angle spinning nuclear magnetic resonance (solid-

15

state

CP-MAS NMR) spectra were recorded on an Agilent DD2 spectrometer (125 MHz).

16

Fourier transform infrared (FTIR) spectra were recorded on a Thermo Fisher 6700 Fourier

17

transform infrared spectroscopy. X-ray photoelectron spectroscopy (XPS) was conducted on a

18

Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer. All XPS data were

19

acquired at a photoelectron takeoff angle of 90°. Thermo advances and XPS peak software were

20

used to calculate elemental compositions and fit high-resolution spectra, respectively.

21

Thermogravimetric analysis (TGA) was conducted on a METTLER TOLEDO TGA/DSC 1

22

equipment. Static contact angle measurements were performed on a Daheng JC2000D1

23

instrument, the results were reported as a mean of at least 4 measurements. The hydrodynamic



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diameter and surface zeta potential were measured using dynamic light scattering technique on a

2

Brookhaven 90 plus PALS particle size analyzer. The silica nanoparticles were dispersed in water

3

by ultrasonication for 20 min at a final concentration of 1 mg/mL, the results were reported as a

4

mean of at least 4 measurements with standard deviation.

5

Preparation of PEG 2000-VS functionalized silica

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The PEG 2000-VS functionalized Stöber silica was prepared using hydroxylated Stöber silica.

7

An amount of 5 mL Stöber silica nanoparticles was dispersed in 100 mL of 95% ethanol by

8

ultrasonication and the pH was adjusted to 4.0 by addition of 1 M nitric acid. The reaction

9

proceeded at 40 °C for 4 h to yield surface silanol group. The resulting silica nanoparticles were

10

collected through centrifugation and washed with acetonitrile. 100 mg of the hydroxylated silica

11

nanoparticles were then dispersed in 10 mL of PEG 2000-VS solution (1 mM in acetonitrile

12

containing 0.1 mM PPh3) by ultrasonication. The mixture was reacted at 60 °C for 4 h with

13

vigorous stirring. After cooling down, the silica particles were collected by centrifugation and

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washed with acetonitrile for 3 times. (Note: a dilute concentration of nanoparticles and plenty of

15

washing were used to improve the accuracy and reproducibility of surface characterization, e.g.,

16

XPS. In practice, 100 mg of the silica nanoparticles can be dispersed in 4 mL of PEG 2000-VS

17

solution for the reaction and almost all the reactants and catalysts could be removed with 1-2 cycles

18

of washing.)

19

The PEG 2000-VS functionalized fumed silica and mesoporous silica were prepared by directly

20

dispersing 100 mg of the silica particles in 10 mL of PEG 2000-VS solution (1 mM in acetonitrile

21

containing 0.1 mM PPh3) by ultrasonication. The mixture was reacted at 60 °C for 4 h with

22

vigorous stirring. After cooling down, the silica particles were collected by centrifugation and

23

washed with acetonitrile for 3 times.


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Preparation of DVS/PVS functionalized silicon wafer and glass

2

The silicon wafer and glass samples were immersed in piranha solution (a 3:1 mixture of 97%

3

sulfuric acid and 30% hydrogen peroxide) for 30 min. (Caution: piranha solution reacts violently

4

with most organic materials and must be handled with extreme care. Note: piranha solution was

5

employed to remove any organic contaminate on the surface in order to improve the accuracy and

6

reproducibility of surface characterization, e.g., XPS. The same level of cleanness could be

7

achieved by plasma cleaning or UV/ozone cleaning.) Once the samples were removed from

8

piranha solution, they were rinsed with copious amount of water and ethanol to remove organic

9

contaminate and then dried under a stream of nitrogen.

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The cleaned silicon wafer and glass samples were immersed in DVS/PVS solutions (200 mM

11

in acetonitrile containing 20 mM PPh3) and reacted at 25 °C for 12 h. After cooling down, the

12

silicon wafer and glass samples were washed with acetonitrile for 3 times before characterization.

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Solvent effect on the stability of Si-O-C bond

14

The solvent effect on the stability of Si-O-C bond was characterized using PEG 2000-VS

15

functionalized Stöber silica nanoparticles. The samples were dispersed in various types of solvents

16

(e. g., methanol, ethanol, isopropanol, acetonitrile, DMSO and water) to a final concentration of 1

17

mg/mL. After storing for different times, the functionalized silica was collected by centrifugation.

18

The stability of Si-O-C bond were characterized using DLS, solid-state 13C CP-MAS NMR and

19

XPS measurements. The PEG 2000-VS functionalized silica stored as powder and pristine silica

20

were also characterized.

21

pH effect on the stability of Si-O-C bond

22

The pH effect on the stability of Si-O-C bond was characterized on DVS/PVS functionalized

23

silicon wafer. The bare silicon wafer and DVS/PVS functionalized silicon wafers were immersed



Page 8 of 34

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in aqueous solution with different pH values (pH 5.0, 7.0 and 9.0) at room temperature for 2 days.

2

The stability of Si-O-C bond was characterized using static water contact angle measurements.

3

Immobilization and controlled release of Cy3

4

100 mg of the silica particles were then dispersed in 10 mL of DVS solution (1 mM in

5

acetonitrile containing 0.1 mM PPh3) by ultrasonication. The mixture was reacted at 60 °C for 4 h

6

with vigorous stirring. After cooling down, the silica particles were collected by centrifugation and

7

washed with acetonitrile for 3 times. The DVS functionalized silica particles were dispersed in

8

Cy3 solution (1 μM in HEPES buffer, pH 8.5) and reacted at room temperature for 4 h. The Cy3

9

functionalized silica particles were collected by centrifugation and washed with acetic buffer (pH

10

4.0) for 3 times. Silica nanoparticles without DVS functionalization were employed as control.

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The silica nanoparticles were dried in an oven and microscopic photographs were taken on an

12

Olympus BX53F fluorescence microscope green light as excitation light and red light as emission

13

light. The fluorescently labeled silica nanoparticles were then dispersed in aqueous solutions with

14

pH 5.0, 7.0 and 9.0. The cumulative release of Cy3 was quantified fluorescently.

15 16

Results and Discussion

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Catalyst screen

18

Using vinyl sulfone group terminated polyethylene glycol 2000 (PEG 2000-VS) as a sample of

19

vinyl sulfone derivative, a series of potential catalysts including 1-methylimidazole (1-MIM),

20

triethylenediamine (DABCO), N,N’-dimethylpyridine (DMAP), triphenyl-phosphine (PPh3),

21

triisopropylphosphine (TIPP) and tricyclohexylphosphine (TCHP) were screened for the

22

PEGylation of silica surface. Using Stöber silica as a model material, nanoparticles were stirred in

23

PEG 2000-VS solution (1 mM in acetonitrile containing 0.1 mM of catalysts) at 60℃ for 4 h. The

24

change in hydrodynamic diameters of the nanoparticles was recorded using DLS measurements


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1

(Figure 1). Among these catalysts, the phosphor-centered catalysts, e.g., PPh3, TIPP, TCHP, all

2

resulted in a 50 nm increase in hydrodynamic diameters in 4 h, exhibiting better catalytic activities

3

than the nitrogen-centered catalysts. This result can be attributed to the higher nucleophilicity of

4

organic phosphines. PPh3 was selected as the catalyst for subsequent experiments since it is

5

conveniently available.

6 7

Figure 1. The hydrodynamic diameter changes of Stöber silica nanoparticles before and after

8

functionalization with PEG 2000-VS using different catalysts.

9

NMR and IR characterization of the surface reaction

10

The surface PEGylation reaction was firstly characterized using solid-state 13C CP-MAS NMR

11

spectroscopy to examine the attachment of the organic moieties, as illustrated in Figure 2a. The

12

PEG 2000-VS functionalized Stöber silica showed a peak at 70 ppm, which can be assigned to the

13

repeated ethylene units in the PEG chains. In contrast, no peaks were observed in the spectra of



Page 10 of 34

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pristine silica and silica treated without catalyst. The results suggest the successful coating of

2

organic moieties via the catalytic reaction.

3 13C

4

Figure 2. (a) solid-state

CP-MAS NMR and (b) FTIR spectra for pristine Stöber silica and

5

PEG 2000-VS functionalized Stöber silica.

6

FTIR was then employed to obtain the chemical information of the nanoparticles before and

7

after PEGylation (Figure 2b). Three main peaks were observed in the FTIR spectrum of pristine

8

silica. The peaks at 3400 cm-1 and 1650 cm-1 can be attributed to the silanol group and water bound

9

through hydrogen bond, respectively.28 The peak at 1050 cm-1 was assigned to the Si-O-Si bond.

10

After functionalization with PEG 2000-VS, no major change was observed at the peak at 1050 cm-

11

1.

12

to consumption of surface silanol groups. New peaks at 2900 cm-1 and 1450 cm-1 were commonly

13

assigned to C-H bond, and the peak at 1290 cm-1 could be assigned to sulfonyl group29. The

14

absorbance around 960 cm-1 was attributed to Si-O-C bond,30 resulting from the reaction of silanol

15

group with vinyl sulfones. Collectively, these results indicate the covalent binding of vinyl

16

sulfones onto the silica surface.

17

XPS characterization

The peaks at 3400 cm-1 and 1650 cm-1 showed significant reduction of intensity, possibility due


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1

X-ray photoelectron spectroscopy (XPS) was used to compare the chemical environment of the

2

functionalized silica surface. The elemental compositions of silica particles before and after

3

functionalization are summarized in Table S1. After functionalization, the amount of C increased

4

significantly while the amount of Si decreased, due to the Si signal attenuation by the PEG chains.

5

Appearance of S signal can be attributed to the sulfonyl group. The result is corroborated by the

6

survey spectra, in which the signals of Si 2s and 2p decreased significantly, while the C 1s signal

7

increased remarkably after functionalization with PEG-VS (Figure 3a). The high-resolution

8

spectra of C 1s and S 2p of silica particles before and after PEGylation were provided in Figure

9

3b and 3c. After functionalization, a new peak appeared at 286.4 eV, corresponding to the carbon

10

atom adjacent to oxygen atom.31 This peak was characteristic of the repeating units of the PEGs.

11

The S 2p spectrum exhibited one distinct peak around 168.0 eV after functionalization, which can

12

be fitted using two S 2p doublets with a 2:1 area ratio and a splitting of 1.2 eV. This peak can be

13

assigned to the introduced sulfone group.32 These XPS spectra further verify the formation of an

14

organic layer on the silica surface after the catalytic reaction.

15



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Figure 3. XPS (a) survey spectra, high-resolution spectra of (b) C 1s and (c) S 2p for pristine

2

Stöber silica (black) and PEG 2000-VS functionalized Stöber silica (red).

3 4

DLS and TEM characterization

5

The attachment of PEG chains was corroborated by the noted differences in the hydrodynamic

6

diameters obtained from DLS measurements (Figure S2). A 50 nm increase is noted after

7

functionalization with PEG 2000-VS. Such a significant size change can be attributed to hydration

8

and stretching of the attached PEG chains in aqueous solutions.33 The surface reaction was also

9

characterized using transmission electron microscope (TEM). No morphological change is

10

observed after the functionalization due to the mild reaction condition (Figure S3). A slight

11

increase in the diameter of silica nanoparticles was also observed, likely due to the dehydration of

12

PEG chains.

13

TGA characterization

14

Thermogravimetric analysis (TGA) was employed to assess the functionalization density of

15

these silica particles, and three types of silica particles, e.g., Stöber silica, fumed silica and

16

mesoporous silica, were compared (Figure S4-S6). For all three types of particles, the TGA curve

17

of pristine silica showed an obvious mass loss at 100 °C, which can be attributed to the absorbed

18

water. The PEG 2000-VS functionalized silica showed another mass loss around 350 °C, due to

19

breakage of Si-O-C bond and the loss of the organic layer. From the TGA curves, the

20

functionalization density can be calculated and the variation among the three types of silica was

21

compared, as summarized in Table S2. The Stöber silica has a functionalization density of only

22

15.2 μmol/g, while the number was calculated to be 114 μmol/g for fumed silica. The low

23

functionalization density of Stöber silica was due to the low surface silanol density and surface

24

area.9, 34-35 The mesoporous silica MCM 41 has a functionalization density of 123 μmol/g. Such a


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1

high value can be attributed to the greater surface area (i.e., around 5 times larger than that of

2

fumed silica and Stöber silica). Nevertheless, the functionalization density based on the catalytic

3

oxa-Michael reaction is much lower than those based on silane treatments,33, 36 likely due to the

4

formation of multilayer structure via silane treatment.

5

Water contact angle and surface potential characterization

6

Other than silica nanomaterials, this reaction can also be applied to bulk silica materials, e.g.,

7

silicon wafer and glass. Using phenyl vinyl sulfone (PVS) as a model compound, the

8

functionalization reaction was characterized by static water contact angle and surface zeta potential

9

measurements. The water contact angle of both silicon wafer and glass showed an increase over

10

30° after functionalization with PVS (Figure S7), indicating successful covalent attachment of

11

hydrophobic phenyl groups on the surface. The surface zeta potential measurements were

12

consistent with the static water contact angle results, as illustrated in Figure S8. Bare silicon wafer

13

is strongly negative charged in neutral solutions because the surface silanol group has an isoelectric

14

point (pI) around 2.37 The surface zeta potential significantly decreased after functionalization with

15

PVS, mainly due to the weak electronegativity of the phenyl surface.

16

Solvent effects on the stability of Si-O-C bond

17

The stability of the resulting Si-O-C bond in different types of solvents including methanol,

18

ethanol, isopropanol, acetonitrile, DMSO, water and solvent-free conditions was investigated

19

using DLS measurements on PEG 2000-VS functionalized Stöber silica nanoparticles. No

20

significant changes in the hydrodynamic diameter of silica nanoparticles were observed when

21

stored either in organic solvents or solvent-free for two weeks (Figure 4a), indicating high stability

22

of PEG 2000-VS functionalized silica in organic solvent or solvent-free conditions. When stored

23

in aqueous solutions, a slow decrease in the hydrodynamic diameter was observed in the first two

24

days, followed by accelerated size reduction as the storage time prolonged (Figure 4b). This can



Page 14 of 34

1

be attributed to the proton sensitivity of Si-O-C bond leading to bond breakage and loss of attached

2

organic moiety. This result is corroborated by the solid-state

3

spectroscopy (Figure S9 and S10). The solid-state 13C CP-MAS NMR signal of PEG chains at 70

4

ppm decreases as the storage prolonged. The organic moiety is almost removed in 4 days. In the

5

XPS survey spectrum (Figure S10a), the decrease of carbon signal was observed. The high-

6

resolution C 1s spectrum (Figure S10b) showed significant decrease in the signal of PEG chains

7

in the first day and little signal remained in 4 days.

13C

CP-MAS NMR and XPS

8 9

Figure 4. The hydrodynamic diameters of (a) PEG 2000-VS functionalized Stöber silica after

10

storage in organic solvents for 2 weeks and (b) PEG 2000-VS functionalized Stöber silica after

11

storage in water and acetonitrile as a function of time. The results were reported as a mean of at

12

least 4 measurements. The polydispersity (PDI) of all the measurements was smaller than 0.15.

13

pH effects on the stability of Si-O-C bond

14

Many studies have shown that the stability of the Si-O-C bonds is related to the experimental

15

conditions such as the pH value38 as well as the chemical group directly attached to the oxygen.39

16

To clarify the effect of pH on the stability of Si-O-C bond, the PEG-VS functionalized silica was

17

dispersed in buffers with different pH values for 2 hours and the hydrodynamic diameter was


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1

measured (Figure 5). Unlike the slow dissolution of pristine silica in basic solutions40, the

2

hydrodynamic diameter of the PEG-VS functionalized silica decreased significantly in basic

3

solutions. The hydrodynamic diameter approximately equals to that of pristine silica when

4

dispersed in solutions with pH = 10 in 2 hours, indicating the total degradation of PEG layers.

5

However, the PEG-VS functionalized silica aggregated in the acid solutions, due to the adsorption

6

of PEG onto silica surface through H bonds that reduced the stability of nanoparticles.36, 41

7 8

Figure 5. The hydrodynamic diameter of PEG 2000-VS functionalized Stöber silica after treated

9

with aqueous solutions with different pH values. The results were reported as a mean of at least 4

10

measurements. The polydispersity (PDI) of all the measurements were smaller than 0.15. * The

11

silica nanoparticles aggregated, and the results reached the detection limit.

12

The effect of attached chemical groups on the degradation behavior was also investigated on

13

bulk silica materials. Silicon wafers functionalized with divinyl sulfone (DVS) and phenyl vinyl



Page 16 of 34

1

sulfone (PVS) were incubated in aqueous solutions with pH values of 5.0, 7.0 and 9.0 for 2 days

2

and the contact angles were recorded (Figure 6). To evaluate the degradation ratio of the functional

3

molecules, the functionalization coverage was calculated from the static contact angle data

4

according to the Cassie equation (equation 1).42

5

cos 𝜃𝑒𝑥𝑝 = 𝜒𝑂𝐻𝜃𝑂𝐻 + 𝜒𝑓𝑢𝑛𝑐𝜃𝑓𝑢𝑛𝑐

(1)

6

where 𝜒𝑂𝐻 and 𝜒𝑓𝑢𝑛𝑐 are the surface coverage of silanol group and functionalization,

7

respectively; 𝜃𝑂𝐻 = 8.3° ± 1.2° is the static contact angle of freshly cleaned silicon wafer; 𝜃𝐷𝑉𝑆

8

= 47.2° ± 1.0° and 𝜃𝑃𝑉𝑆 = 50.1° ± 1.4° are the static contact angle values of DVS and PVS

9

functionalized silicon wafer, respectively.

10 11

Figure 6. Water contact angles of divinyl sulfone (DVS) and phenyl vinyl sulfone (PVS)

12

functionalized silicon wafers before and after treatment with glycine buffer (pH 5.0), phosphorus

13

buffer (pH 7.0) and carbonate buffer (pH 9.0) for 2 days.

14

After immersed in neutral solutions for 2 days, DVS functionalized silicon wafers have a

15

functionalization coverage of 96%, while the data of PVS functionalized silicon wafer is only 72%.

16

Both DVS and PVS functionalized silicon wafers showed significant decrease in water contact

17

angle after treated with acidic or basic solutions compared to the neutral solutions, because acid


Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

and base could catalyze the breakage of Si-O-C bond.43 When immersed in either acid or basic

2

solutions for 2 days, functionalization coverage of DVS functionalized silicon wafer reduced

3

~45%, which is consistent with the reported acid/base catalytic Si-O-C bond breakage. PVS

4

functionalized silicon wafer showed ~96% reduction in functionalization coverage after immersed

5

in acid solutions for 2 days. The discrepancy between DVS and PVS can be mainly attributed to

6

the difference in the chemical structure. The low functionalization density derived from steric

7

hindrance of PVS also contributes to the high degradation rate. In all, the stability of Si-O-C bonds

8

can be affected by the terminal group of vinyl sulfone molecules, functionalization density on

9

surface as well as solution conditions.

10

Immobilization and controlled release of Cy3

11

Using DVS as a crosslinker, various functional molecules can be immobilized on silica surface

12

due to the versatile reactivity of vinyl sulfone group towards -SH, -NH2 and -OH,31 which would

13

broaden the applications of functional silica-based materials (Scheme 2). DVS has been reported

14

as a crosslinker for the anchoring natural and synthetic polymers onto silica surface through

15

multiple-step methods.44-47 Previously, it has been reported that silica materials were firstly treated

16

with amino group terminated silane (e.g., 3-aminopropyltriethoxysilane), followed by reaction of

17

DVS with amino group in aqueous solutions. However, amino group terminated silane treatments

18

would lead to drastic change of surface charge. Moreover, the unpredictable multilayer

19

functionalization of silane would probably cause crosslinking of DVS and increase the particle

20

size.17 The catalytic surface reaction facilitates the direct attachment of DVS without drastically

21

changing the surface charge and particle size, which is advantageous over the multiple-step

22

methods.



Page 18 of 34

1 2

Scheme 2. Molecules being interested are immobilized and sequentially released in a controllable

3

manner onto DVS functionalized silica materials.

4

Choosing Cy3 as a model functional molecule, the silica particles were further labeled

5

fluorescently (Figure S11). Compared to the unobservable fluorescence of the silica nanoparticles

6

without crosslinker, strong fluorescence was observed, indicating the successful attachment of Cy3.

7

To establish the release of fluorescence molecules in a controllable manner, the functionalized

8

silica nanoparticles were dispersed in aqueous solutions with pH values of 5.0, 7.0 and 9.0. The

9

excitation and emission spectra of released molecules is consistent with Cy3 (Figure 7a). A first-

10

order release curve was observed for the release of fluorescence molecules in all pH conditions as

11

shown in Figure 7b. The increase of pH value could accelerate the release of Cy3. These

12

phenomena can be attributed to the breakage of Si-O-C bond. In addition, the electrostatic

13

repulsions between the silanol group and the Cy3 terminated sulfonic group would also accelerate

14

the release of fluorescence molecules. Considering the versatile reactivity of vinyl sulfone group

15

towards thiol, amine and alcohols, a variety of functional molecules, e.g., drugs or biomolecules,


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1

could be immobilized on the silica surface. Controlled release can be realized on silica materials

2

by simply tuning the solution conditions.

3 4

Figure 7. (a) Excitation and emission spectra of Cy3 and released molecules. (b) Cumulative

5

release of fluorescence molecules from Cy3 labeled silica in aqueous solutions with different pH

6

values.

7

Conclusion

8

To summarize, we reported the oxa-Michael reaction of inorganic silanol groups with vinyl

9

sulfones, which was utilized in the functionalization of silica-based materials with sizes ranging

10

from nanoscale (including Stöber silica, fumed silica and mesoporous silica) to macroscale

11

(including silicon wafer and glass). Compared to alcohol- or organophosphonate-based strategies,

12

the reaction condition for the one-step solution reaction temperature is rather mild

13

omission of pretreatment and inert gas protection makes our strategy more convenient than Si-H

14

bond reactions24. While the traditional silane strategy always involves a dilute solution (1%-5%,

15

v/v) to reduce the tendency to auto-polymerization of silanes and formation of multilayer

16

structure17, the chemical stability of vinyl sulfones towards hydrolysis and polymerization allows

17

high reactant concentrations. Furthermore, the reactant solutions can be collected and reused,

18, 22.

The



Page 20 of 34

1

which may significantly reduce the solution volume. In all, intrinsic stability of vinyl sulfones,

2

easy operation and mild reaction temperature make our strategy a promising way for functioning

3

silica-based materials.

4

Using DVS as a crosslinker, versatile functional molecules can be immobilized onto silica

5

materials, facilitating the fabrication of silica nanosystems or biosensors displaying designed

6

functions. The stability of resulting Si-O-C bond can be tuned by the chemical structure of the

7

vinyl sulfone compounds as well as the environmental factors. Increase in the hydrophobicity and

8

functionalization density of the vinyl sulfone compounds could improve the stability of Si-O-C

9

bonds. The controlled degradation of Si-O-C bond in aqueous solutions allows the controllable

10

release of attached molecules, indicating applications in carries for drugs and catalysts.

11

Acknowledgement

12

This work was in part supported by the National Natural Science Foundation of China (Nos.

13

21773022 and 31771033) and Fundamental Research Funds for the Central Universities

14

(DUT16RC(3)019 and DUT17RC(3)021). Fang Cheng is grateful for the open grant provided by

15

the Key Laboratory for Ultrafine Materials of the Ministry of Education. Wei He and Bingbing

16

Sun thank the Recruitment Program of Global Youth Experts for support. The authors are grateful

17

to Dr. Qingqin Ge and Mr. Ting Wang at Thermo Fisher Scientific (China) Co. Ltd. for the XPS

18

measurements and helpful discussion.

19

Supporting Information. The supporting information includes elemental compositions from XPS

20

analysis, TEM photographs, hydrodynamic diameters and TGA curves of pristine silica and PEG

21

2000-VS functionalized silica, 13C CP-MAS NMR spectra and XPS characterization for stabilities

22

of Si-O-C bond, micrographs of Cy3 functionalized silica materials. This material is available free

23

of charge via the Internet at http://pubs.acs.org.


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pre-activated material for immobilization of biomolecules. J Mater Chem 2010, 20 (34), 71897196.

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Insert Table of Contents Graphic and Synopsis Here

19 20 21

An efficient strategy for functionalization of silica materials with tunable release behavior of tailed molecules.


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Functionalization of silica surface via (a) traditional strategies and (b) the catalytic oxa-Michael reaction of surface silanol groups with vinyl sulfones. 88x67mm (600 x 600 DPI)

ACS Paragon Plus Environment


The hydrodynamic diameter changes of Stöber silica nanoparticles before and after functionalization with PEG 2000-VS using different catalysts. 88x65mm (600 x 600 DPI)


Page 26 of 34

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(a) solid-state 13C CP-MAS NMR and (b) FTIR spectra for pristine Stöber silica and PEG 2000-VS functionalized Stöber silica. 177x71mm (600 x 600 DPI)



XPS (a) survey spectra, high-resolution spectra of (b) C 1s and (c) S 2p for pristine Stöber silica (black) and PEG 2000-VS functionalized Stöber silica (red). 177x87mm (600 x 600 DPI)


Page 28 of 34

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The hydrodynamic diameters of (a) PEG 2000-VS functionalized Stöber silica after storage in organic solvents for 2 weeks and (b) PEG 2000-VS functionalized Stöber silica after storage in water and acetonitrile as a function of time. 177x70mm (600 x 600 DPI)



The hydrodynamic diameter of PEG 2000-VS functionalized Stöber silica after treated with aqueous solutions with different pH values. 88x66mm (600 x 600 DPI)


Page 30 of 34

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Water contact angles of divinyl sulfone (DVS) and phenyl vinyl sulfone (PVS) functionalized silicon wafers before and after treatment with glycine buffer (pH 5.0), phosphorus buffer (pH 7.0) and carbonate buffer (pH 9.0) for 2 days. 177x65mm (600 x 600 DPI)



Molecules being interested are immobilized and sequentially released in a controllable manner onto DVS functionalized silica materials. 88x53mm (600 x 600 DPI)


Page 32 of 34

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(a) Excitation and emission spectra of Cy3 and released molecules. (b) Cumulative release of fluorescence molecules from Cy3 labeled silica in aqueous solutions with different pH values. 177x69mm (600 x 600 DPI)



An efficient strategy for functionalization of silica materials with tunable release behavior of tailed molecules. 83x47mm (600 x 600 DPI)


Page 34 of 34

New Strategy for Functionalization of Silica Materials via Catalytic Oxa

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