Covalent bonding of grafted polymer brushes of poly(poly(ethylene glycol) monomethacrylate) on surface of silicon quantum dots and the activation of the end hydroxyls

2.1 Materials

Hydrofluoric acid (HF, 40%), nitric acid (HNO₃, 67%), hydrogen peroxide (H₂O₂, 30%), silver nitrate (AgNO₃, 99.8%), 10-undecen-1-ol (UO, 98%), 2-bromo-2-methylpropyl bromide (BMPB, 98%), cuprous chloride (98.5%), cupric chloride (98.5%), 2,2′-bipyridine, N-hydroxysuccinimide (NHS, 99%), N,N′-dicyclohexylcarbodiimide (DCC, ⩾99%), 4-dimethylaminopyridine (DMAP, 98%), succinic anhydride (98%), Tetrahydrofuran (THF, 99.0%) and bovine serum albumin (BSA, ∼66 kD, Biological Reagent) were purchased from Aladdin Reagent (Shanghai) Company. PEGMA (MW 360 Da) was obtained from Aldrich Company. All the reagents were used as received. Doubly distilled water was used throughout the work. The silicon slices ((1 1 1) orientation, thickness: 450 ± 50 μm, p-type, boron doped, electrical resistivity: 8–13 Ω cm) were cleaned thoroughly in piranha solutions (H₂O₂ (30 wt% in H₂O)/H₂SO₄ (98 wt%) 1:3 (v:v)) at 150 °C for more than 2 h to remove any fouling and were washed with copious water and dried with nitrogen.

2.2 Surface modifications on Si/SiQDs with UO and successively with BMPB

Method of preparing SiQDs on a silicon slice was described in our previous report (Liu et al., 2014), as was depicted in (1) of Fig. 1. Surface Si-H_x (x = 1, 2 or 3) were readily produced when the Si/SiQDs slice was disposed with 0.05 M HF solutions for 1 min as described in (2). Such a Si/SiQDs slice with surface hydrides (Si/SiQDs-H) was washed completely with water and was dried with streams of nitrogen gases. Then it was put into a vessel containing neat 5 mL UO. The reaction system was tighten with an oil-seal device and bubbled with nitrogen for at least 10 min to wipe out the atmosphere. The hydrosilylation was kept at 140 °C for 4 h. Then it was cleaned by immersing it in THF for 5 min for three times to remove any adsorbed UO and dried with flowing nitrogen for obtaining Si/SiQDs-UO as described in (3). Finally, the Si/SiQDs-UO slice was immersed in anhydrous THF solutions containing 0.5 mL BMPB at room temperature for 3 h for realizing the esterification reaction between the end hydroxyls of Si/SiQDs-UO and BMPB. The slice was washed with THF for three times to obtain the surface-initiated Si/SiQDs (denoted with Si/SiQDs-UO-BMPB) as described in (4).

2.3 Constructing grafted polymer brushes of P(PEGMA) on Si/SiQDs surface and the activation of the side chains

Polymer brushes on surface of such initiated SiQDs could be constructed via atom transfer radical polymerization. Firstly, the catalysts composed of 20 mg CuCl₂, 40 mg CuCl and 15 mg 2,2′-bipyridine were put into 20 mL water. Then the monomer of 0.5 mL PEGMA was added in and mixed completely. The Si/SiQDs-UO-BMPB slice was immersed in this mixture. Before heating, the system was flown with nitrogen to exhaust air of the reaction system. The reaction proceeded at 40 °C for 4 h as described in (5) of Fig. 1. The slice was washed with copious water to wipe out the absorbed inorganic and organic foulings to obtain surface-grafted P(PEGMA) on Si/SiQDs slice (denoted with Si/SiQDs-UO-BMPB-g-P(PEGMA)). Such a slice was input into THF solutions containing 50 mg succinic anhydride and 20 mg DMAP at 40 °C for 2 h to introduce carboxyl groups at the grafted side chains of the polymer brushes, as described in Supporting Information (SI) of Fig. S1(1). Then it was immersed in THF solutions of 80 mg NHS and 20 mg DCC at 40 °C for 4 h to transfer the carboxyl groups into NHS esters depicted in Fig. S1(2) of SI for activation of the side chains of the surface polymer brushes. Finally, the slice was washed with copious anhydrous alcohol and dried with blowing of nitrogen gases.

BSA solutions (2.00 mg mL⁻¹ in phosphate buffer saline of pH = 7.2) of 10 mL were mixed with such NHS esters-activated SiQDs obtained by an ultrasonic dispose of an activated Si/SiQDs-UO-BMPB-g-P(PEGMA) slice. And the mixtures were incubated for 4 h at 25 °C. Then they were employed in an experiment of SDS-PAGE gel electrophoresis.

2.4 Characteristic techniques

The SiQDs solution was obtained by ultrasonic treatment of a SiQDs/Si slice and its PL was measured by a fluorescence spectrometer (LS 45, PerkinElmer). The SiQDs were also detected by high-resolution transmission electron microscopy (HR-TEM, Tecnai G2 F20 S-TWIN, FEI Company) and atomic force microscopy (AFM, Veeco Instruments Inc.) with tapping mode in air at room temperature and analyzed using the Nanoscope software. The stepwise modifications on the SiQDs/Si were evaluated by infrared (IR) spectra (Nicolet 380, Thermo Nicolet Corporation) and X-ray photoelectron spectra (XPS, KRATOS AMICUS). The XPS was recorded by K-Alpha with a monochromatized Al Kα X-ray source. Survey with a binding energy of 0–1350 eV along with elemental information with high-resolution scans was utilized to prove the surface stepwise reactions through analyses on the changes of surface components.

3.1 Surface modification on the Si/SiQDs slice with UO

The SiQDs can be generated on the silicon substrate via two-step etching on a silicon wafer. A porous silicon slice is prepared at first with Ag-assisted chemical etching. Further etching with a mixture of HF/HNO₃ brings the SiQDs into being on the silicon substrate, which was proved by Fig. 2a. The Si/SiQDs slice emits magnificent red lights when exposed under ultraviolet lamp (wavelength 365 nm). Surface SiQDs can be isolated from the silicon substrate by an ultrasonic dispose. PL measurement of the SiQDs in water is displayed in Fig. 2b. And its emission wavelength is located at 610 nm. The mixture also emits red fluorescence when irradiated by a portable ultraviolet lamp. The PL peak is consistent with the photograph of the PL shown in the insert of Fig. 2b, which is a convenient evidence of judging the generation of the SiQDs on a porous silicon slice.

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Figure 2

The photoluminescence (PL) of a Si/SiQDs slice is revealed in photograph of (a). The PL spectrum and photograph of the SiQDs in water are displayed in (b).

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The SiQDs are also witnessed by HR-TEM and AFM images, shown in Fig. 3. Fig. 3a displays the clear crystalline planes of the SiQDs and the distance is about 0.31 nm. Fig. 3c and d exhibits the sizes of about 5 nm of the SiQDs. On account of the difference of the etching velocities at difference crystalline planes the size uniform of the SiQDs may be worse than that from the method of chemical reduction of silane, derived from the splitting of the Si-halogen bonds and the stacking of silicon atoms during the reduction (Cheng et al., 2012; Portolés et al., 2012). Judged from the formation mechanism, chemical reduction brings about more homogenous SiQDs. However, we provide an inexpensive and convenient approach of preparing SiQDs herein. Fig. 3a and b also exhibits the relative homogenous SiQDs.

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Figure 3

HR-TEM and AFM images of the SiQDs obtained by an ultrasonic dispose of a Si/SiQDs slice corresponding to (a) and (b) respectively. The section analysis and the depth histogram of (a) are shown in (c) and (d) respectively.

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Hydrides on silicon surface (Si—H_x, x = 1, 2 or 3) are readily produced when a wafer or a porous silicon slice is immersed in aqueous HF or ammonia fluoride solutions. Moreover, the Si—H_x species possess high reactivity with some organic compounds containing terminal vinyl or acetenyl through additive reactions. This provides a particularly favorable condition for the chemical modification on solid silicon materials. The surface SiQDs-H_x are demonstrated by IR of Fig. 4a. Peaks of 2086, 2113 and 2136 cm⁻¹ (Li and Buriak, 2006) of Fig. 4a are arisen from the Si—H, Si—H₂ and Si—H₃ species respectively which marked with a shadow area. Such surface hydrides behave actively in combination with vinyl group of UO demonstrated by IR spectrum of Fig. 4b and XPS of Fig. 5a.

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Figure 4

IR spectra of the Si/SiQDs-H, Si/SiQDs-UO, Si/SiQDs-UO-BMPB and Si/SiQDs-UO-BMPB-g-P(PEGMA) slices corresponding to (a–d) respectively.

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Figure 5

XPS with narrow scans of C 1s of the Si/SiQDs-UO, Si/SiQDs-UO-BMPB and Si/SiQDs-UO-BMPB-g-P(PEGMA) slices corresponding to (a), (c) and (d) respectively. The Br 5d5/2 spectra of the Si/SiQDs-UO-BMPB slice are also exhibited in (b).

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Bands of 2926 and 2854 cm⁻¹ in Fig. 4b are attributable to the asymmetrical and symmetrical vibration of C—H in methylene groups of surface hydrocarbon chains. Peak 3347 cm⁻¹ should arise from the stretching vibration of O—H (v(O—H)) of the terminal hydroxyl groups. We cannot detect any C—C signal in IR spectrum of Fig. 2b which discloses that it is the vinyl group reacts with surface Si—H_x. And UO molecules are attached with surface Si covalently by Si—C bonds. The side reaction between Si—H_x and —OH of UO is scarcely observed in this case (Dusciac et al., 2007). So the Si/SiQDs are chemically modified by UO while the terminal —OH remains intact. The peripheral hydroxyl groups of the SiQDs facilitate the further surface modifications. Such a hydrosilylation gives rise to the drop in the intensity of the peak Si—H_x in Fig. 4a. C 1s core-level spectrum of Fig. 5a also demonstrates the introduced carbon chains. In Fig. 5a, the integrated area of peak 284.8 eV and 286.4 eV should be assigned to C—C—C and C—C—O components respectively (Guo et al., 2009). The former is exactly ten times of the latter which is in complete agreement with the theoretical atomic ratio. Such XPS analyses agree with the results of IR measurements, which also prove the successful attachment of UO onto the Si/SiQDs slice. Most of all, such a reaction has not changed the PL of the SiQDs.

3.2 Surface initiation and construction of grafted polymer brushes of P(PEGMA)

The peripheral hydroxyl groups of Si/SiQDs-UO slice are apt to be esterified by BMPB which is a conventional reagent of activating hydroxyls in atom transfer radical polymerization (Xu et al., 2009a,b). Fig. 4c proves such an esterification of the end hydroxyls. The newly occurred peak of 1737 cm⁻¹ should be assigned to stretching vibration of C⚌O in the resultant esters. Meanwhile, the intensity of v(O—H) in Fig. 4c becomes weaker compared with that of Fig. 4b. XPS of Fig. 5b illustrates the signal of Br 3d (He et al., 2008; Xu et al., 2004). High-resolution of C 1s core-level spectrum in Fig. 5c also indicates that the tert-butyl bromide is introduced onto surface of the SiQDs. According to the chemical structure of surface hydrocarbon chains as shown in Fig. 1, there are four C-contained species with a different ambience in a chain. Areas of peak 284.8 eV and 286.4 in Fig. 5(c) should be due to the C 1s core-level spectra of C—C—C and C—C—O species which is in accordance with the C 1s analyses on Si/SiQDs-UO slice shown in Fig. 5a. Area of peak 286.0 eV should be attributed to C—C—Br species (He et al., 2008) while 289.0 eV belongs to C 1s core-level spectrum of O—C⚌O components (Imanishi et al., 2008). Therefore the XPS of C 1s along with the IR spectra demonstrates the surface esterification between the peripheral hydroxyls and BMPB, which indicates successful immobilization of the initiators on the surface of Si/SiQDs-UO slice. Additionally, PL of the SiQDs does not to be damaged during the esterification, which is checked with irradiation of an ultraviolet lamp.

Such BMPB-initiated surface facilitates the grafted polymerization of PEGMA on surface of a Si/SiQDs-UO-BMPB slice for obtaining Si/SiQDs-UO-BMPB-g-P(PEGMA). Due to the SiQDs are fixed on silicon substrate the isolation of the excess reactants becomes rather easy. Washing or immersing in water can remove the excess materials and surface foulings. The intensive absorptions of v(O—H) and alkyl chains marked in Fig. 4d suggest the grafted P(PEGMA) chains on SiQDs surface. The band 1735 cm⁻¹ should be due to the esters deriving from the overlap of the grafted side chains and isobutyrate esters. XPS of Fig. 5d also demonstrates such a grafted polymerization of PEGMA. High-resolution of C 1s spectrum can be curve-fitted into four components having binding energies at about 284.8, 286.0, 286.3 and 288.6 eV. Areas of peak 284.8 eV and 286.0 eV are still assigned to C—C—C and C—C—Br respectively. Peak 286.3 eV should belong to the polymerized unit of (OCH₂CH₂O) (Xu et al., 2005) of the monomer. The integrated area of peak 288.6 eV should be comprised mainly of O⚌C—O in the side chains (Xu et al., 2005). Based on these analyses the grafted polymer brushes of P(PEGMA) have been synthesized successfully on surface of the Si/SiQDs-UO-BMPB slice. Besides, we can hardly observe the signal of Si 2p when measuring XPS of the Si/SiQDs-UO-BMPB-g-P(PEGMA) slice, which is also observed in our previous work (Liu et al., 2011). This may be resulted from dense organic chains on the surface of Si/SiQDs slice.

The PL stability of the SiQDs is vital for their applications. Surface passivation on SiQDs is beneficial for remaining their PL characters as have been stated above. We find that the Si/SiQDs-UO and Si/SiQDs-UO-BMPB-g-P(PEGMA) slices still emit a dazzling fluorescence even it is placed in atmosphere for more than a month when irradiated under an ultraviolet lamp. The PL spectra of the aqueous solutions of SiQDs-UO and SiQDs-UO-BMPB-g-P(PEGMA) obtained by exfoliating them from the Si/SiQDs-UO and Si/SiQDs-UO-BMPB-g-P(PEGMA) slices respectively are shown in Fig. 6a and b. Obviously, chemical modifications do not alert their PL properties, as is shown in the inset of Fig. 6a and b. However, the fluorescent emission shifts from 618 nm of (a) to 582 nm of (b). This may be attributed to the dense grafted polymer brushes on surface of the SiQDs, which expands the energy level difference between excited state and ground state.

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Figure 6

Photographs of the Si/SiQDs-UO and Si/SiQDs-UO-BMPB-g-P(PEGMA) slices as well as the aqueous solutions of SiQDs-UO and SiQDs-UO-BMPB-g-P(PEGMA) are displayed in (a) and (b) respectively. Such performances are consistent with PL spectra of (a) and (b).

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3.3 Introduction of active NHS esters on the terminals of the grafted P(PEGMA) brushes

Now that the grafted polymer brushes are constructed on surface of the SiQDs successfully and PL stability of such modified SiQDs in atmosphere is still remained, if the reactivity of the side chains is enhanced the modified SiQDs are supposed to become reactive. This will expand applications of the SiQDs. The carboxyl groups can be introduced onto the ends of the side chains via the reaction between the end hydroxyls of the grafted polymer brushes and succinic anhydrides, described in Fig. S1(1) of SI. And finally the reactive NHS esters are produced readily at the ends of the side chains under rather mild conditions depicted in Fig. S1(2) of SI. IR spectra of Fig. 7 demonstrate such changes during these chemical modifications. Band 1731 cm⁻¹ of Fig. 7a should be due to the overlap of stretching vibration of C⚌O groups resulted from ester and carboxyl. The characteristic peaks of 1731, 1785 and 1815 cm⁻¹ of Fig. 7b suggest formation of the end NHS esters, assigned to the stretching vibration of C⚌O of ester, symmetrical and asymmetrical stretching vibration of succinic ring respectively (Chen et al., 2009; Voicu et al., 2004). Such tripartite peaks of IR signals are regarded as an evidence of generating NHS esters in many previous reports (Liu et al., 2006; Xiao et al., 2004). Since such reactions occur on the terminals of the side chains the density of the active NHS esters will therefore be improved. That provides more opportunities of realizing the linkages between the modified SiQDs and target molecules.

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Figure 7

Carboxyl groups are introduced onto surface of a Si/SiQDs-UO-BMPB-g-P(PEGMA) slice, giving IR spectrum of (a). These —COOH are activated finally with NHS bringing IR spectrum of (b).

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The obtained surface NHS esters are reactive with BSA, which was depicted in Fig. S2 of SI. The images obtained from the gel imaging system prove the reactivity of the end NHS esters. The mixtures of BSA and activated SiQDs show fluorescent phenomenon exhibited in (b) and (d) while (a) and (c) of the sole BSA solutions manifest no such performance. It is obvious that the modified SiQDs and BSA are bonded covalently. And this demonstrates the reactivity of the end NHS esters of the side chains.