A reversible molecule-gated system using mesoporous silica nanoparticles functionalized with K⁺-stabilized G-rich quadruplex DNA

Abstract

This paper proposed a novel and reversible molecule-gated system consisting of mesoporous silica nanoparticles (MSN) functionalized on the pore outlets with a G-rich quadruplex DNA (GQDNA). In this system, K⁺-stabilized GQDNA as a molecular switch was grafted onto the MSN surface through the covalent cross-linking approach. In the absence of silver ion (Ag⁺), GQDNA could fold into a quadruplex structure through π–π stacking between G–G pairs mediated by potassium ions (K⁺), thus blocking the pore outlets and inhibiting the release of entrapped guest molecules. In the presence of Ag⁺, the Ag⁺ can interact with G bases, leading to the unfolded GQDNA and subsequently opened pores. Interestingly, the opened pore mouths can be closed again by introducing glutathione (GSH) molecules, which can bind competitively with Ag⁺ ions by the thiols to result in the conformation transition of DNA from unfolded structures to quadruplex structures. By the simple conformational changes of GQDNA gatekeepers, the molecular gate can switch reversibly by the alternate addition of Ag⁺ ions and GSH molecules. As a proof-of-concept, Ru(bipy)₃²⁺, a strong fluorescence dye molecule, was loaded into GQDNA grafted MSN (MSN-Ru-GQDNA). The result showed that the MSN-Ru-GQDNA has a highly reversible ability to open/close pores which was proved by the released percentage of Ru(bipy)₃²⁺. With these excellent features, the release of Ru(bipy)₃²⁺ can be easily controlled at will. We believed that further developments of this reversible molecule-gated system will provide a promising nanodevice for on-demand molecular transport.

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Introduction

Owing to the stable mesostructure, large surface areas, large loading capacity, good biocompatibility, and chemically modifiable surfaces, mesoporous silica nanoparticle (MSN) has drawn great attention in the design of stimuli-responsive molecule-gated systems.^1–3 With these excellent attractive features, MSN has made itself a hopeful and wide platform for various applications including sensing,⁴ catalysis,⁵ and controlled release.^6,7 Of the MSN-based controlled release systems previously studied, diverse classes of “smart” stimuli-responsive gatekeepers, such as organic molecules,⁸ inorganic nanoparticles,⁹ and supramolecular nanovalves,^10,11 have employed to construct the molecule-gated systems for preventing model molecules from escaping the pore system and then releasing the entrapped molecules by a range of external triggers including pH,^12,13 redox reagent,^14,15 enzyme,¹⁶ thermal¹⁷ and optical.¹⁸ Among these gatekeepers, nucleic acids, employed as the building material for nanotechnology and materials science,¹⁹ have attracted ever-increasing attention in constructing the molecule-gated systems due to their characteristics of robust physicochemical nature, programmable sequence-specific recognition, and conformational polymorphism.^20,21 For example, Chang et al.²² selected a double-stranded oligonucleotides to block the pores of MSN, and then unlocked the pores by the thermal-stimulated dissociation of the dsDNA. Also, Climent et al.²³ employed an oligonucleotides as the cap to lock the pores through the electrostatic interaction between MSN and oligonucleotides. And the pores can be then opened by a highly effective displacement reaction in the presence a target complementary strand. Most of these blocked pores in the reported nucleic acids-gated systems have usually unlocked by removing relative nucleic acids strands capped on the pores of MSN.^24,25 However, the opened pores of these nucleic acids-gated systems cannot return to the closed state again, thus limiting its application in a lot of fields,²⁶ such as on-demand molecular transport. Therefore, a new strategy was needed to overcome these issues for designing a reversible molecule-gated system for the application of on-demand molecular transport.

In recent years, a variety of nucleic acids have been selected as the gatekeepers to construct the reversible molecule-gated systems. The reversible ability in almost all of these systems has been driven via the conformational switch of nucleic acids trigged by different stimuli, such as pH,^27,28 light,^29,30 and ion.³¹ However, the nucleic acids-based reversible molecule-gated mechanism was still in the incipient development stage. There was also has a potential developed area of reversible molecule-gated systems for on-demand molecular transport. It was widely reported that guanine-rich (G-rich) nucleic acid sequences, acting as an important role in biology, were existed in the region of gene promoter and chromosome telomeres.^32,33 In the presence of metal ions, such as potassium ion (K⁺), the G-rich nucleic acid sequences can fold in a unique higher-order G-quadruplexes structure by Hoogsteen-type base pairing.³⁴ With these characteristics, more and more G-quadruplexes-based systems have been constructed to achieve various applications, such as sensors³⁵ and detection.³⁶ Recent studies by other groups have reported that the structure of G-quadruplexes can be unfolded via the strong interaction between silver ion (Ag⁺) and guanosine bases (G bases). Based on these properties, Zhou et al.³⁷ has developed a simple Ag⁺ detection method by disrupting the structure of G-quadruplex-hemin DNAzymes through the interaction between Ag⁺ and guanosine bases.

Therefore, inspired by the above findings and efforts, we have sought to take advantages of these unique features of MSN and G-quadruplexes to rationally design a novel class of reversible molecule-gated system that enable to on-demand transport loaded guest molecules. The schematic was illustrated in Fig. 1. For achieving the molecule-gated system with a smart reversible function, a K⁺-stabilized G-rich quadruplex DNA (GQDNA), employed as the gatekeeper, was grafted on the surface of MSN through the click chemistry between alkynyl and azide groups to obtain MSN-GQDNA. In the absence of Ag⁺, GQDNA could form a quadruplex structure through π–π stacking between G–G pair mediated by K⁺, thus blocking the pore outlets and inhibiting the leakage of the entrapped guest molecules. In the presence of Ag⁺, G bases have interacted with Ag⁺, leading to the unfolded GQDNA and subsequently opened pores. More especially, after treatment with thiol-containing molecules, the Ag⁺ has separated from G bases and then bond to the thiol groups, resulting in the refolded GQDNA and the reclosed pores. Thus, the closed and opened states of pores could be mediated by alternating addition of Ag⁺ and thiol-containing molecules. In order to prove the feasibility of this principle, Ru(bipy)₃²⁺ and glutathione (GSH), as the model molecule and thiol-containing stimulus respectively, were used to investigate the reversible ability of this smart molecule-gated system.

Fig. 1 Schematic illustration of the reversible molecule-gated system using MSN functionalized with K⁺-stabilized GQDNA.

Experimental section

Materials

Sodium hydroxide (NaOH), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), tert-butyl alcohol (tBuOH), toluene and tetraethylorthosilicate (TEOS, 28%) were obtained from Xilong reagent company (Guangdong, China). Glutathione (GSH), [Ru(bipy)₃]Cl₂ (bipy = 2,2′-bipyridine) and tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine were obtained from Sigma-Aldrich (USA). 3-chloropropyltrimethoxysilane (ClTMS), CuBr (99.9%) and N-cetyltrimethylammonium bromide (CTAB) were purchased from Alfa Aesar. Silver nitrate (AgNO₃), trihydroxymethylaminomethane (Tris-base), acetic acid (HAc), ascorbic acid and sodium azide (NaN₃, 99%) were purchased from Dingguo reagent company (Beijing, China). Sodium chloride (NaCl) and potassium chloride (KCl) were obtained from Taishan chemical plant (Guangdong, China). Disodium hydrogen phosphate (Na₂HPO₄), monopotassium phosphate (KH₂PO₄) and potassium acetate (KAc) were purchased from Guanghua chemical plants (Shantou, China). Hydrochloric acid (HCl), ethyl alcohol and methyl alcohol were obtained from Ante Biochemical Co., LTD (Anhui, China). Nanopure water (18.2 MΩ; Millpore Co., USA) was used in all experiments and to prepare all buffers. All the chemicals were used as received without further purification. The oligonucleotides were obtained by Sangon Biotechnology Inc. (Shanghai, China). The sequences are as follows: 5′-CHCH-TGG GTA CGG GTT GGG AAA-3′ (G-rich quadruplex DNA), 5′-CHCH-AAA AAA AAA AAA AAA AAA-3′ (DNA1).

Characterization

Transmission electron microscopy (TEM) image was obtained on a JEOL 3010 microscope with an accelerating voltage of 100 kV. Fourier transform infrared (FTIR) spectra were obtained from a TENSOR 27 spectrometer, Bruker Instruments Inc., Germany. The ζ potential of MSN materials were measured by dynamic light scattering (DLS), Malvern Inc., England. Small-angle powder X-ray diffraction patterns of the MSN were obtained from a Scintag XDS-2000 powder diffractometer, using Cu Kα irradiation (λ = 0.154 nm). N₂ adsorption–desorption isotherm was obtained at 77 K on a Micromeritics ASAP 2010 sorptometer by static adsorption procedures. Samples were degassed at 373 K and 10⁻³ Torr for a minimum of 12 h prior to analysis. Brunauer–Emmett–Teller (BET) surface area was calculated from the linear part of the BET plot according to IUPAC recommendations. Pore size distribution was estimated from the adsorption branch of the isotherm by the Barrett–Joyner–Halenda (BJH) method. UV-vis spectra were collected by using a DU-800. All fluorescence spectra were measured on a Hitachi F-7000 FL spectrophotometer.

Preparation and surface-activation of mesoporous silica nanoparticle (MSN)

Preparation of MSN was performed according to the sol–gel process. First, N-cetyltrimethylammonium bromide (CTAB, 0.50 g) was dissolved in 240 mL of pure water and then 1.75 mL of 2 M NaOH was added. After adjusting the solution temperature to 80 °C, 2.50 mL of TEOS was then added dropwise into mixture slowly under vigorously stirring condition and continually reacted for 2 h to give rise to white precipitates. Subsequently, the obtained white precipitates were separated by centrifugation, and washed several times by ethanol and DI water respectively. The purified MSN was dried in high vacuum container at −60 °C. The surfactant templates in the pore channels of MSN were then removed by stirring as-synthesized MSN at 80 °C in 50 mL of anhydrous ethanol containing 0.50 mL HCl (37.2%). After 24 h stirring, the resulting surfactant-free MSN was washed with water and ethanol, and then placed under high vacuum at −60 °C to remove the remaining solvent from the mesopores. Then, the surfactant-free MSN was modified with ClTMS to yield the chlorine-modified MSN (MSN-Cl). In this process, 0.4 mL of 3-chloropropyltrimethoxysilane (ClTMS) was added into 40 mL of anhydrous toluene containing 0.40 g of surfactant-free MSN. The resulting mixed solution was continuously stirred for 24 h at 90 °C. The resulting material was centrifuged and extensively washed with nanopure water and ethanol, and then dried under high vacuum container at −60 °C. Finally, the obtained MSN-Cl was further activated with sodium azide to get azide-functionalized nanoparticles (MSN-N₃). 0.20 g of as-synthesized MSN-Cl sample was added into a saturated solution of sodium azide in 20 mL DMF solution and stirred at 90 °C for 6 h. The obtained material was then collected by centrifugation, and redispersed in PBS buffer solution for 6 h under stirring to remove remaining DMF from the mesopores of MSN-N₃. Subsequently, the MSN-N₃ was washed several times with DI water and ethanol, and then dried on vacuum container at −60 °C.

Preparation of GQDNA-grafted MSN containing Ru(bipy)₃²⁺

5.0 mg MSN-N₃ was dispersed into 1.0 mL of Ru(bipy)₃²⁺ solution (1 mM) by shaking 24 h at room temperature. Then Ru(bipy)₃²⁺-loaded MSN-N₃ was collected by centrifugation, and washed with DI water three times, and stored in 1.5 mL Tris–HAc buffer. In order to obtain the G-Quadruplex DNA (GQDNA) grafted Ru(bipy)₃²⁺-loaded MSN-N₃, 300 μL alkyne-modified single-stranded GQDNA (100 mM), 2 μL tris(benzyltriazolylmethylamine) ligand (0.1 M in DMSO/tBuOH 3 : 1) and 2 μL CuBr solution (0.1 M in DMSO/tBuOH 3 : 1) were added into 300 μL stock solution of Ru(bipy)₃²⁺-loaded MSN-N₃, which was then shook overnight at room temperature. Subsequently, the particles were separated by centrifugation and washed with Tris–HAc buffer to remove residual GQDNA and physisorbed Ru(bipy)₃²⁺ from the exterior surface of the material, and dried under high vacuum to yield the GQDNA grafted Ru(bipy)₃²⁺-loaded MSN-N₃ (MSN-Ru-GQDNA). The DNA1 grafted Ru(bipy)₃²⁺-loaded MSN-N₃ (MSN-Ru-DNA1) was obtained according to the description above mentioned.

Reversible investigation of MSN-Ru-GQDNA

The capping behavior of this system was first investigated. 0.3 mg of MSN-Ru-GQDNA, MSN-N₃-Ru and MSN-Ru-rDNA particles was placed in a cuvette respectively, which was then carefully filled with 200 μL of Tris–HAc buffer (pH 7.0, K⁺: 20 mM). The fluorescence intensity of released Ru(bipy)₃²⁺ was monitored every 20 minutes. In order to study the uncapping behavior of this GQDNA-gated system, 2.0 mg of MSN-Ru-GQDNA was first dispersed into 1.5 mL Tris–HAc buffer (pH 7.0, K⁺: 20 mM). Then 100 μL different media (For Tris–HAc: pH 7.0, Ca(Ac)₂, Mg(NO₃)₂, Mn(Ac)₂, Co(Ac)₂, AgNO₃: 1.5 mM) was respectively added into 200 μL above solution of MSN-Ru-GQDNA. After 4 h incubation, the suspensions of these samples were collected by centrifugation and then measured via the fluorescence intensity of Ru(bipy)₃²⁺ molecules by fluorescence emission spectroscopy. Another experiment was used to further investigate the release behavior of MSN-Ru-GQDNA under different concentration of Ag⁺. The Tris–HAc buffer (200 μL, pH 7.0, K⁺: 20 mM) containing 0.2 mM, 0.3 mM and 0.5 mM Ag⁺ respectively was carefully filled into the cuvette containing 0.3 mg of MSN-Ru-GQDNA. All of Ru(bipy)₃²⁺ released from the pore voids to the solution was monitored through fluorescence intensity of Ru(bipy)₃²⁺ molecules by fluorescence emission spectroscopy (λ_ex = 452 nm, λ_em = 610 nm).

As for on-demand molecular release, MSN-Ru-GQDNA (1 mg) was dispersed in 200 μL of Tris–HAc buffer (pH 7.0, K⁺: 20 mM) containing 0.5 mM of Ag⁺. The suspension were collected every 30 min over a range of 480 min, and the amount of released Ru(bipy)₃²⁺ were drove by carrying out interconversion cycles of the opening and closing states via the alternate treatment of Ag⁺ and GSH. Moreover, the renewability of MSN-Ru-GQDNA system was also studied. First, 200 μL of Tris–HAc buffer (pH 7.0, K⁺: 20 mM) containing 0.5 mM of Ag⁺ was added into 1.0 mg of MSN-Ru-GQDNA. After 5 h incubation, the sample was collected by centrifugation and then added into 200 μL of Ru(bipy)₃²⁺ solution (5 mM) for 5 h incubation. Subsequently, this sample was also collected by centrifugation and continuous alternately treated by the above method. All the supernatants were monitored via the fluorescence intensity of Ru(bipy)₃²⁺ molecules by fluorescence emission spectroscopy (λ_ex = 452 nm, λ_em = 610 nm).

Results and discussion

Synthesis and characterization of MSN

For the design of the reversible molecule-gated system, two components were chosen, namely inorganic nanoparticles supporter and the renewable gatekeepers. In this work, because of its various advantageous properties, MCM-41-type MSN was selected as the inorganic nanoparticles supporter for constructing this reversible molecule-gated system. The MSN with a MCM-41-type mesoporous structure was first synthesized by the sol–gel method and then functionalized with a chlorine group on the surface of MSN. The obtained particles (MSN-Cl) were characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and N₂ adsorption–desorption measurements. As shown in Fig. 2a, the as-synthesized MSN-Cl with the fine spherical shape had the good monodispersity and uniform size. The diameter of the spherical MSN-Cl, which had the typical hexagonally arranged pores, was about 160 nm (Fig. 2b). The dynamic light scattering (DLS) assay indicated that the MSN-Cl with a polydispersity index (PDI) of 0.083 was 190 nm, which was attributed to the swell of nanoparticles in aqueous solution than that in a monolayer in air by TEM.

Fig. 2 SEM images (a) and TEM images (b) of MSN-Cl.

The XRD pattern of MSN-Cl (Fig. 3a) showed three well-resolved diffraction peaks that could be assigned as (100), (110) and (200) Bragg peaks with an a₀ cell parameter of 4.1 Å, which was consistent with the characteristic diffraction pattern of MCM-41 type MSN. Moreover the N₂ adsorption–desorption isotherms of the as-synthesized MSN-Cl exhibited a characteristic type IV curve with a specific surface area of 851.2 m² g⁻¹, an average pore diameter of 2.2 nm, and a narrow pore distribution, which further indicated the typical MCM-41-type structure of MSN-Cl (Fig. 3b). The characteristics of the as synthesized MSN were all summarized in Table S1.†

Fig. 3 Powder X-ray pattern (a) and nitrogen sorption isotherms (b) of as-synthesized MSN-Cl. Inset: pore size distribution plots of MSN-Cl.

Functionalization of MSN

In order to eventually achieve the reversible molecule-gated system, the as-prepared MSN-Cl was first treated with sodium azide in N,N-dimethylformamide (DMF) to give an azide-functionalized surface (MSN-N₃). The change of zeta potential of MSN before and after functionalization has revealed this modified process. The obtained MSN-N₃ was showed a negative zeta potential of −36.9 mV, which was lower than that of MSN (Fig. S1†). This result might be attributed to the ionization of azide group modified in the surface of MSN. Subsequently, Ru(bipy)₃²⁺, a strong fluorescence dye molecules, was selected as the model molecule to encapsulate into the pores of MSN-N₃. By measuring with UV-vis spectroscopy and calculating with the standard curve of Ru(bipy)₃²⁺ (Fig. S2†), The loading amount of Ru(bipy)₃²⁺ was determined to be about 96.55 μmol g⁻¹ SiO₂. After azide activation and Ru(bipy)₃²⁺ load, a G-rich quadruplex DNA (GQDNA), act as the reversible gatekeeper, was then grafted onto the surface of MSN-N₃ by a click chemistry approach to block the pores of MSN-N₃ and reserve loaded Ru(bipy)₃²⁺, giving rise to the Ru(bipy)₃²⁺-loaded GQDNA-grafted MSN (MSN-Ru-GQDNA). The formation of GQDNA grafted MSN-N₃ was first confirmed by TEM images. As shown in Fig. S3,† The MSN-N₃ was covered a thin layers, which was the most direct evidence to prove the modifying of the GQDNA around the MSN matrix. The zeta potential of MSN-Ru-GQDNA also showed the successful modification of GQDNA in this grafting process. Compared with MSN-N₃, the value of zeta potential of MSN-GQDNA was changed into −27.2 mV, which was attributed to the decreased azide group and grafted GQDNA all around MSN (Fig. S1†).

Moreover, the successful surface functionalization of MSN was illustrated by the Fourier Transform Infrared (FTIR) in Fig. 4. Compared with MSN-Cl which only has the framework vibrations of silica, MSN-N₃ displayed an apparent characteristic azide stretching signal at 2110 cm⁻¹ (see the dotted line in Fig. 4). After further attachment with GQDNA on the MSN-N₃, the absorption band at 2110 cm⁻¹ was strongly reduced in intensity, which clearly indicated the successful binding behavior of GQDNA onto the surface of MSN-N₃ by covalent conjugation. The immobilization efficiency of GQDNA tethered to the surface of MSN was calculated indirectly through the quantification of left GQDNA after the process of graft. The left DNA was measured and calculated by the Biospec-nano UV-vis spectroscopy to be 4.39 nmol, which corresponded to an immobilization amount of 3.68 μmol g⁻¹ SiO₂. It was worth note that the immobilization amount of DNA1 was calculated to be 3.39 μmol g⁻¹ SiO₂. In addition, to further investigate the dispersity of MSN-GQDNA, the size distribution of MSN-N₃ and MSN-GQDNA was measured by DLS. As displayed in Fig. S4,† MSN-GQDNA has an average diameter of 223.7 nm, which was larger than that of MSN-N₃ (195.7 nm). This result was attributed to the grafted GQDNA on the surface of MSN-N₃. In addition, the PDI of MSN-N₃ and MSN-GQDNA were 0.142 and 0.075 respectively, which demonstrated a good dispersity of these two nanoparticles.

Fig. 4 FTIR spectra of the samples before and after modification: MSN-Cl, MSN-N₃, and MSN-GQDNA.

Capping and uncapping studies of MSN-Ru-GQDNA

After confirming the successful modification of GQDNA onto the MSN-N₃, the capping and uncapping behaviour of this system was investigated. The fluorescence intensity of released Ru(bipy)₃²⁺ in the supernatant was measured by fluorescence emission spectroscopy at 610 nm (λ_ex = 452 nm). As shown in Fig. 5a, the release amount of Ru(bipy)₃²⁺ from MSN-Ru-GQDNA can be ignored in the presence of potassium ions (K⁺). However, about 85.9% and 82.2% of the Ru(bipy)₃²⁺ were released from MSN-N₃-Ru and MSN-Ru-DNA1 in the presence of K⁺. These excellent release percentages of Ru(bipy)₃²⁺ from MSN-N₃-Ru and MSN-Ru-DNA1 were attributed to the unclosed pores of MSN. All of results clearly demonstrated an excellent retention efficiency of Ru(bipy)₃²⁺ in the pores of MSN by virtue of the blocking behaviour of GQDNA through π–π stacking between G–G pair mediated by K⁺. According to the finding displayed above, the excellent blocking and storage effect of GQDNA was obtained.

Fig. 5 Release profiles of Ru(bipy)₃²⁺ from different nanoparticles in the presence of K⁺ (a). The specificity response of MSN-Ru-GQDNA under different ions (for Tris–HAc (control): pH 7.0, Ca(Ac)₂, Mg(NO₃)₂, Mn(Ac)₂, Co(Ac)₂, AgNO₃: 1.5 mM) (b).

After proving the capping behaviour of GQDNA, the uncapping behaviour of this GQDNA-gated system was further investigated. It was reported that the structure of G-quadruplexes can be unfolded via the strong interaction between silver ion (Ag⁺) and guanosine bases (G bases). Therefore, a stimuli-responsive experiment with different ions was carried out to study the “smart” opening ability of pores. As illustrated in Fig. 5b, after treating with different ion for 4 h, only Ag⁺ induced a dramatic increase in the amount of released Ru(bipy)₃²⁺ from MSN-Ru-GQDNA, whereas no obvious changes of released Ru(bipy)₃²⁺ were observed in the presence of other ions. The good selectivity of this stimuli-responsive system was mainly attributed to the special interaction between Ag⁺ and G bases. With these unique characteristics, an Ag⁺-responsive GQDNA-gated system was obtained.

Subsequently, the release statuses of MSN-Ru-GQDNA under different concentration of Ag⁺ were investigated. As displayed in Fig. 6, the release of Ru(bipy)₃²⁺ from MSN-Ru-GQDNA was Ag⁺ dependent. The release percentage of Ru(bipy)₃²⁺ was approximately 30.5%, 53.8%, and 82.3% within 4 h at concentration of Ag⁺ in 0.2 mM, 0.3 mM and 0.5 mM respectively. This result further proved that the release of Ru(bipy)₃²⁺ from MSN-Ru-GQDNA was mainly relied on the interaction between Ag⁺ and G bases and subsequently unfolded GQDNA. All of above results suggested that the Ru(bipy)₃²⁺ could be efficiently confined in the pores using GQDNA as gatekeeper, and fast released upon the Ag⁺-triggered unfold of GQDNA. These characters made the MSN-Ru-GQDNA effective to be a “smart” molecular-gated system.

Fig. 6 Release profiles of Ru(bipy)₃²⁺ from the MSN-Ru-GQDNA under different concentrations of Ag⁺.

Reversible behaviour of MSN-Ru-GQDNA

Moreover, the distinctive advantage of this system, which has the reversible ability in capping and uncapping pores, was further studied. As a proof of concept, a partial release of trapped Ru(bipy)₃²⁺ could be regulated with open-close cycles by alternating addition of Ag⁺ and GSH. As demonstrated in Fig. 7a, the closed state at starting 60 min strongly constrained the release of Ru(bipy)₃²⁺. However, an obvious release of the entrapped Ru(bipy)₃²⁺ molecular was triggered in the open state from 60 min to 120 min as a result of the conformational change of GQDNA when the Ag⁺ was added (see the arrow in Fig. 7a). By treating with GSH at 120 min, the release of the entrapped Ru(bipy)₃²⁺ was again restricted, which was attributed to the interaction between GSH and Ag⁺ and subsequently refolded GQDNA. At 180 min, Ag⁺ was added and further release of entrapped Ru(bipy)₃²⁺ occurred until GSH was again added at 240 min, inhibiting the release of Ru(bipy)₃²⁺. By continuously alternating Ag⁺ and GSH, the release of Ru(bipy)₃²⁺ can be easily controlled on-demand. The decreased Ru(bipy)₃²⁺ release percentage in each open segment was attributed to the reduced amount of Ru(bipy)₃²⁺ which released from the pores in each cycle. The results demonstrated that the interconversion ability of GQDNA was reversible and that the transportation of the Ru(bipy)₃²⁺ in small portions could be operated at will through alternatively adding Ag⁺ and GSH in this system. To further validate the reversible behaviour of MSN-Ru-GQDNA, cysteine (Cys), another thiol-containing molecule, was employed as the stimuli to construct this reversible system. It can be seen from Fig. S5† that the release of loaded Ru(bipy)₃²⁺ also could be regulated with open-close cycles by alternating addition of Ag⁺ and Cys. This result further proved that this reversible behaviour of MSN-Ru-GQDNA could be obtained via alternatively adding Ag⁺ and thiol-containing molecule in this system.

Fig. 7 Partial molecular release profile of Ru(bipy)₃²⁺ from MSN-Ru-GQDNA as a function of Ag⁺ and GSH alternative addition. (a). Renewability of MSN-Ru-GQDNA system (b).

The renewability of MSN-Ru-GQDNA system was illustrated in Fig. 7b. After 5 h loading, the fluorescence intensity of MSN-Ru-GQDNA was increased obviously, indicating a successful loading of Ru(bipy)₃²⁺ (see the point 1 in Fig. 7b). By incubating with Ag⁺ for 5 h, the fluorescence intensity of MSN-Ru-GQDNA was decreased, which was attributed to the release of trapped Ru(bipy)₃²⁺ (see the point 2 in Fig. 7b). Excitedly, we found that Ru(bipy)₃²⁺ can be reloaded into MSN-Ru-GQDNA after releasing trapped Ru(bipy)₃²⁺ from the pores of MSN-Ru-GQDNA (see the point 3 in Fig. 7b). More important, the fluorescence intensity of MSN-Ru-GQDNA after Ru(bipy)₃²⁺ loading has a negligible change in subsequently two cycles. Therefore, we speculated that this loading–releasing cycle of MSN-Ru-GQDNA can be continuously repeated for several times. All of these consequences clearly demonstrated that this renewable molecular-gated system of MSN-Ru-GQDNA was a potential system for on-demand molecular transport.

Conclusions

In summary, we have demonstrated a novel and reversible molecule-gated system to on-demand transport guest molecular. Through the simple and rapid synthesizing, this K⁺-stabilized GQDNA-gated reversible system can effectively block the pore outlets and inhibit the release of entrapped guest molecules by the folded GQDNA through π–π stacking between G–G pair. In the presence of Ag⁺, the pores of GQDNA-gated system can be opened via the interaction between Ag⁺ and G bases, leading to the release of loaded guest molecular. Moreover, by alternating treatment with Ag⁺ and thiol-containing molecule, this GQDNA-gated system has displayed an excellently reversible ability. As a proof-of-concept, Ru(bipy)₃²⁺, act as the guest molecular, was first loaded into GQDNA-gated system to demonstrate this reversible process. These features of MSN-Ru-GQDNA for reversible delivery of loaded guest molecule provide a significant advance toward deeper understanding and exploration of reversible molecule-gated system for some fields, such as medicine and environmental protection.

Acknowledgements

This work was supported in part by the Project of Natural Science Foundation of China (Grants 21175039, 21322509, 21305035, 21190044, and 21221003).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: The dynamic light scattering (DLS) assay, standard curve of Ru(bipy)₃²⁺, the N₂ adsorption–desorption isotherms characteristics of as synthesized MSN-Cl. See DOI: 10.1039/c5ra15016g

‡ These authors contributed equally.

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