Article Cite This: Langmuir 2018, 34, 4036−4042
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Multifunctional Surface Modification of Nanodiamonds Based on Dopamine Polymerization Yun Zeng,†,∥ Wenyan Liu,‡,∥ Zheyu Wang,§ Srikanth Singamaneni,§ and Risheng Wang*,† †
Department of Chemistry and ‡Center for Research in Energy and Environment, Missouri University of Science and Technology, Rolla, Missouri 65409, United States § Department of Mechanical Engineering and Materials Science, Institute of Materials Science and Engineering, Washington University in St. Louis, St Louis, Missouri 63130, United States S Supporting Information *
ABSTRACT: Surface functionalization of nanodiamonds (NDs), which is of great interest in advanced material and therapeutic applications, requires the immobilization of functional species, such as nucleic acids, bioprobes, drugs, and metal nanoparticles, onto NDs’ surfaces to form stable nanoconjugates. However, it is still challenging to modify the surface of NDs due to the complexity of their surface chemistry and the low density of each functional group on the surfaces of NDs. In this work, we demonstrate a general applicable surface functionalization approach for the preparation of ND-based core−shell nanoconjugates using dopamine polymerization. By taking advantage of the universal adhesion and versatile reactivity of polydopamine, we have effectively conjugated DNA and silver nanoparticles onto NDs. Moreover, the catalytic activity of ND-supported silver nanoparticle was characterized by the reduction of 4-nitrophenol, and the addressability of NDs was tested through DNA hybridization that formed satellite ND−gold nanorod conjugation. This simple and robust method we have presented may significantly improve the capability for attaching various functionalities onto NDs and open up new platforms for applications of NDs.
1. INTRODUCTION Nanodiamonds (NDs), a new type of carbon nanoparticles, have attracted considerable attention recently due to their great promise in a broad range of applications, particularly in materials science and biomedicine.1−3 For example, it has been demonstrated that NDs are outstanding catalyst supports for metal nanoparticles because of their high surface area, superchemical stability, thermal conductivity, electrical insulating, and strong metal−support interaction features.4,5 In addition, the excellent biocompatibility and nontoxicity of the NDs have extended their capabilities for bioimaging/sensing and biomedical carriers for delivering molecules, such as proteins, ligands, nucleic acids, antigens, and drugs, into biological systems.6−12 To efficiently execute these applications, the conjugation of those functional species on the surfaces of NDs is an essential prerequisite.13,14 However, the low density of each type of functional groups (e.g., carboxyl, hydroxyl, lactone, etc.) on the surface of NDs15 makes them extremely challenging for modification to form stable nanohybrids, either by covalently attaching a dense layer of functional ligands/ biomolecules or by reducing uniformed metal nanoparticles on their surfaces. Up to now, various methods have been proposed and applied to modify the surface of NDs, such as covalent linkage of peptide nucleic acids (PNA) to the surface of the ND via amide bond formation,10 surface functionalization of the ND by DNA through click chemistry,8 and noncovalent © 2018 American Chemical Society
wrapping of proteins or polymer derivatives on the ND surface.7,16−20 Nevertheless, those strategies can be time consuming with multistep procedures, as well as being inefficient and highly specific to each reaction. Therefore, it is extremely desirable to develop a simple and generally applicable strategy to surface-functionalize the NDs, which can meet the diverse demands for surface grafting of varied functionalities without specific chemical modification of each species. Dopamine (DA),21 a natural neurotransmitter in the brain, exhibits unique properties that provide the ability to overcome the aforementioned challenge. It can self-polymerize and spontaneously form a polydopamine (PDA) layer atop virtually any solid surface under a mild alkaline condition (pH ∼ 8.5). Such layers contain amino groups and phenolic groups, which can be further utilized to facilely immobilize thiol/aminecontaining biomolecules and directly reduce metal nanoparticles in an aqueous solution.22,23 Owing to its simplicity and versatility, this PDA-assisted functionalization method has been used to modify various nanoparticles (e.g., gold nanoparticles (AuNPs),24 Fe3O4,25 SiO2,26 polymeric nanoparticles,27,28 graphene,29 and mosaic virus30) for numerous applications since its development. However, PDA-assisted Received: February 13, 2018 Revised: March 11, 2018 Published: March 12, 2018 4036
DOI: 10.1021/acs.langmuir.8b00509 Langmuir 2018, 34, 4036−4042
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was further demonstrated by attaching gold nanorods (AuNRs) to NDs to form satellite assemblies. The present method can be facilely applied to immobilize other molecules on the NDs to develop ND-based novel catalysts, biosensors, and nanocarriers.
surface functionalization of NDs has rarely been reported. For example, Boukherroub and co-workers reported the functionalization of hydroxylated NDs with dopamine derivatives, including triethylene glycol and azide (−N3), enhanced ND’s solubility, and the possibility of postmodification via “click” chemistry.20,31 We hypothesize that the versatile coating capability of PDA can be used to simplify the surface modification of NDs, eliminating the inefficiency and complexity involved in the traditional ND modification process. Herein, we have exploited the efficient surface functionalization of NDs with DNA oligomers and silver nanoparticles to form functional nanoconjugates via a PDA-based surface modification method. As illustrated in Scheme 1, these
2. EXPERIMENTAL SECTION 2.1. Materials. DNA strands were purchased from IDTDNA, the sequences are listed in Supporting Information. Monocrystalline nanodiamonds (20 nm) were purchased from Adámas Nanotechnologies, Inc., and 50 and 100 nm monocrystalline nanodiamonds were purchased from FND Biotech, Inc., Taiwan. Dopamine was purchased from Sigma-Aldrich, Co. AgNO3, NH3·H2O, 1.0 M of Tris− HCl buffer, and other inorganic reagents were purchased from Fisher Scientific, Co. All of the chemicals were directly used without further purification. 2.2. Preparation of PDA Coating on ND Surface. To prepare PDA−ND nanoparticles, NDs were mixed with a freshly prepared DA solution (in 0.01 M Tris buffer, pH = 8.5) with varied concentrations of 30, 50, 75, and 100 μg/mL and allowed to process for 12 h under vigorous stirring at room temperature. Then, the resulting PDA−ND nanoparticles were collected by centrifugation (16 000g, 2 h) and washed with Milli-Q water three times at a centrifugation speed of 16 000g for 1 h. Finally, the nanoparticles were dispersed in Milli-Q water for further use. 2.3. Synthesis of AgNP−PDA−NDs. The presynthesized PDA− ND nanoparticles were first mixed with various concentrations of [Ag(NH3)2]+ (0.08, 0.16, 0.24, 0.4, and 0.6 mg/mL) to make up 100 μL total volume, followed by sonication for 10 min. Then, the resultant AgNP−PDA−NDs were separated from free silver ion by centrifugation at 16 000g for 15 min and subsequently washed three times with Milli-Q water. Finally, the products were dispersed in 100 μL of Milli-Q water. In addition, the different reaction times were tested on 20 nm PDA−NDs from 10 to 60 min (10, 20, 30, and 60 min). 2.4. Surface Functionalization of PDA−ND with DNA and Its Conjugation with AuNRs. Thiolated DNA strands (1000 μM; 25 μL) were first reduced by tris(2-carboxyethyl) phosphine hydrochloride at room temperature for 2 h, followed by subsequent filtration through a G-25 column (GE Healthcare) to remove small molecules. Then, the active thiol-DNA strands were mixed with PDA−ND nanoparticles in a Tris−HCl (0.01 M; pH = 8.5) solution with vigorous stirring overnight. After reaction, the resulting DNA−PDA− ND nanoconjugates were collected by centrifugation at 16 000g for 2 h, rinsed twice with 200 μL of 150 mM NaCl to remove excessive DNA, and then resuspended in the same NaCl solution for further use. The conjugation of AuNRs on the DNA−PDA−ND nanoconjugates was accomplished by complementary hybridization of DNA strands coated on both nanoparticles. To facilitate the hybridization, the particle mixture was annealed from 45 to 20 °C with speeds of 45−35 °C, 1 °C/h and 35−20 °C, 2 °C/h. In addition, the control experiment was prepared under the same conditions but PDA−ND nanoparticles were treated with non-thiol-DNA. The hybridization of NDs and AuNRs was confirmed by SEM and TEM. 2.5. Catalytic Reduction of (4-NP) with AgNP−PDA−ND Nanoparticles. The catalytic activities of synthesized AgNP−PDA− ND nanoconjugates (NDs, 100 nm; DA, 60 μg/mL; [Ag(NH3)2]+, 0.4 mg/mL) were investigated to determine the reduction of 4-NP into 4AP in the presence of NaBH4. In a typical reaction, 250 μL of 0.1 M freshly prepared NaBH4 was added in the solution containing 2.5 mL of 0.1 mM 4-NP. Subsequently, 10 μL of a prepared AgNP−PDA− ND suspension was added into the above solution and the reaction started immediately. The UV−vis absorption of the solution was performed to monitor the reduction progress, with a scanning range from 250 to 650 nm. The kinetic rate constant of the reduction process was determined by measuring the change in the absorbance at 400 nm as a function of time. 2.6. Dynamic Light Scattering (DLS). DLS analyses were conducted using a Malvern Zetasizer instrument (Zetasizer ZS90, Malvern, U.K.). The assays were performed under thermostatic
Scheme 1. Schematic Illustration of the Surface Functionalization of NDs with Polydopamine (PDA) and the Formation of AgNP−PDA−NDs and Core−Satellite Assembly of AuNR−(DNA−PDA−ND) Cluster through DNA Hybridization
nanoconjugates were prepared using a two-step process. First, the ND nanoparticles were coated with a thin PDA layer by dopamine oxidative self-polymerization in a Tris buffer solution (pH 8.5). Then, these PDA-coated NDs were either covalently functionalized with DNA to form DNA−PDA−ND nanoconjugates via the Michael addition reaction21 or used to absorb and reduce [Ag(NH3)2]+ ions into metallic nanoparticles on their surfaces to form AgNP−PDA−ND nanoconjugates. We systematically investigated the influence of the concentrations of DA on the thicknesses of PDA layers and the concentration of [Ag(NH3)2]+ on the size of silver nanoparticles. This was confirmed by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and dynamic light scattering (DLS). In addition, the catalytic activity of the synthesized AgNP−PDA−NDs was checked by catalytic reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP), and the DNA-directed surface addressability 4037
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Figure 1. Characterization of the thickness of a PDA layer on the surface of 100 nm NDs. (A) The size distribution of uncoated NDs and their corresponding TEM image. (B−D) The size distribution of PDA−ND nanoparticles with varied concentrations of dopamine (50, 75, and 100 μg/ mL) and their corresponding TEM images, respectively. The inset photography shows the colorimetric change in the corresponding samples. conditions (25 °C) and with an equilibration time of 120 s before each measurement. 2.7. Ultraviolet−Visible (UV) Spectra. The UV spectra of AgNP−PDA−NDs were measured using a UV spectrophotometer (Ultrospec 9000, GE Healthcare) with a 1 cm quartz cuvette at wavelengths between 250 and 550 nm with 1 nm intervals. 2.8. Scanning Electron Microscopy (SEM). The particle morphologies were characterized using a S4700 scanning electron microscope (Hitachi, Japan), operated at 5 kV accelerating voltage. The SEM samples were prepared by depositing 1−2 μL of samples on silicon substrates, which were cleaned with a piranha solution (H2SO4/ H2O2 = 3:1 (v/v)) for at least 30 min and subsequently washed with ethanol and Milli-Q water. The sample substrates were then sequentially dipped into 50% ethanol (v/v) for 10 s, 80% ethanol (v/v) for 50 min, and then washed with absolute ethanol to remove any buffer salts. Finally, the samples were air-dried overnight before SEM imaging. 2.9. Transmission Electron Microscopy (TEM). A Tecnai F20 field-emission electron microscope was used to observe a series of NDs, PDA−NDs, AgNP−PDA−NDs, and AuNRs−PDA−NDs at an acceleration voltage of 200 kV. The TEM samples were prepared by dropping 5 μL of solution onto carbon film-coated Cu TEM grids,
which were glow-discharged by a plasma cleaner. Then the grids were washed with Milli-Q water two times and air-dried at room temperature. To analyze the elements of the nanostructures, energy dispersive X-ray spectrometry (EDS) was used with a Li-drift Si detector and 134 eV energy resolution.
3. RESULTS AND DISCUSSION NDs of various sizes (20, 50, and 100 nm) were employed in this study to test the feasibility of the proposed surface functionalization method. TEM and SEM images, as shown in Figures 1A and S1, respectively, reveal that uncoated NDs appear to be unstable with respect to aggregation and spontaneously formed microclusters, thereby limiting their practical applications. To improve their dispersion stability and simultaneously facilitate their subsequent functionalization, the NDs were added to a slightly alkaline Tris buffer solution of dopamine and vigorously stirred for 12 h. It was observed during the coating process that the color of ND suspensions gradually changed from colorless to dark, and the higher the concentration of DA, the darker the solution became, indicating 4038
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Figure 2. Characterization of AgNP−PDA−NDs. (A) TEM images of AgNP−PDA−NDs and the size distribution of AgNPs by adding 0.4 mg/mL of [Ag(NH3)2]+, and 0.6 mg/mL of [Ag(NH3)2]+ for (B). The average sizes of AgNPs are ∼24 and ∼28 nm, respectively.
experimental conditions. Further, an investigation was made of the impact of the concentration of [Ag(NH3)2]+ on the formation of AgNPs on the PDA−ND surface. Taking 100 nm NDs as an example, when the concentration of [Ag(NH3)2]+ was as low as 0.08 mg/mL, only a few of the nanoparticles were attached on the PDA−ND surface. With an increase in the [Ag(NH3)2]+ concentration to 0.24 mg/mL, the number and size of AgNPs increased accordingly (Figure S6). When the [Ag(NH3)2]+ concentration increased further, as seen in Figure 2, more and larger AgNPs were formed on the PDA−ND surface with average sizes of ∼24 and ∼28 nm that corresponded to the [Ag(NH3)2]+ concentrations of 0.4 and 0.6 mg/mL, respectively. Moreover, the influence of PDA shell thickness on AgNP reduction efficiency was also examined. The results showed that the thinner layer of PDA resulted in a fewer number of nanoparticles being reduced on the PDA−ND surface, which was likely due to nonsufficient catechol groups on the PDA layer for reduction of the silver precursors (Figure S7). AgNPs, owing to their surface plasmon vibrations, exhibited strong absorbance in the visible region of around 400 nm.36 Thus, UV−vis measurements were also made to characterize the formation of AgNPs on the PDA−ND surface. Figure 3A illustrates the UV−vis spectra of AgNP−PDA−NDs with an increased concentration of [Ag(NH3)2]+ from 0.08 to 0.6 mg/ mL. It is clear that with a low concentration of [Ag(NH3)2]+ (0.08 mg/mL), no obvious peak was observed. However, as the [Ag(NH3)2]+ concentration was increased, it was observed that a broad absorbance peak appeared at ∼400 nm, suggesting the formation of Ag nanoparticles on the PDA−ND surface. With further increase in the concentration of [Ag(NH3)2]+, the intensity of the peaks increased and the peaks were red-shifted, indicating an increase in the nanoparticle size. It can be seen that the spectra of AgNPs, reduced at 0.4 and 0.6 mg/mL of [Ag(NH3)2]+, exhibited maximum absorbance at 410 and 430 nm, corresponding to AgNPs with diameters of ∼20 and ∼30 nm, respectively. This was in line with the TEM observation. To further confirm the composition of the prepared AgNP− PDA−NDs, additional analysis was performed using energy dispersive X-ray spectrometry (EDS, Figure 3B). The EDS result reveals that elements present in the AgNP−PDA−NDs were mainly carbon and silver, thus confirming the existence of reduced silver nanoparticles on the PDA−ND surface.
oxidation-induced self-polymerization of DA (inset of Figure 1). The TEM images, as can be seen in Figure 1B−D, clearly demonstrate that a thin ring formed around the NDs, which confirmed that a uniform PDA coating successfully formed on each ND surface. By varying the concentration of DA, the thickness of the PDA layer could be easily controlled. On the basis of the TEM image analysis (Figures 1B−D and S2), the thickness of the PDA layers on 100 nm NDs increased with DA concentration, from ∼5 to ∼10 nm and ∼15 nm, corresponding to the final DA concentrations of 50, 75, and 100 μg/mL, respectively. Consistent with the TEM results, DLS measurements also showed that the hydrodynamic diameter of the PDA-coated NDs had increased along with the increases in DA concentration (left column, Figure 1B−D). In addition, we also performed the surface coating of PDA on the smaller NDs. The results showed that the thickness of the PDA layer could reach ∼3−4 and ∼5 nm when 20 and 50 nm NDs were treated with 30 and 50 μg/mL of DA, respectively (Figures S3 and S4B). After a prime coating of the NDs with PDA, we then investigated the deposition of AgNPs on the PDA-coated NDs via a PDA-induced reduction of silver ions. It has been generally accepted that catechol groups in PDA play a central role in the PDA-assisted metallization process. This can induce the formation of nanoparticles upon the reduction of metal precursors and immobilize them on a PDA-coated surface.32−35 As such, after the PDA−NDs were mixed with silver nitrate solution, with the assistance of sonication, the silver ions were in situ gradually reduced into AgNPs and immobilized on the PDA−ND surface, forming AgNP−PDA−ND nanoconjugates. As evidenced by the TEM images shown in Figure 2, AgNPs grew effectively and were distributed on the surface of the PDA−NDs without notable aggregation, indicating that NDs provide excellent support for metal nanoparticles. To investigate and determine an optimum reaction time, the reduction reaction was performed using 20 nm PDA−ND at various time durations, from 10 to 60 min, with a constant concentration of [Ag(NH 3 ) 2 ] +. TEM observations (as presented in Figure S5) reveal that a 10 min reduction time was sufficient for forming uniformly sized AgNPs on the PDA− ND surface, whereas an extended reduction time led to the appearance of AgNPs that were nonuniform in size. Thus, 10 min was chosen as the optimal reaction time for the 4039
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Figure 4. Time-dependent evolution of UV−vis spectra showing the catalytic reduction of 4-NP into 4-AP by AgNP−PDA−NDs.
platform for secondary surface modifications of NDs. The conjugation reaction between NDs and SH-DNA was carried out in the 0.01 M Tris buffer (pH 8.5) for 12 h with vigorous stirring. To confirm the DNA conjugation on the NDs, we performed hybridization experiments between the prepared DNA−PDA−NDs and gold nanorods (AuNRs, ∼50 nm in length) covered by corresponding complementary thiolated DNA strands. As can be seen in Figure 5A, upon hybridization,
Figure 3. Characterization of reduced AgNPs on the surface of NDs. (A) UV−vis spectra showed the formation of AgNPs on the ND’s surface with various concentrations of silver solution (0.08−0.60 mg/ mL). (B) EDS spectrum of AgNP−PDA−NDs.
After successfully forming AgNP−PDA−ND nanoconjugates, catalytic activity was further studied by employing catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction, in the presence of excessive NaBH4. The characteristic absorption peak of the mixture of 4NP and NaBH4 was observed at 400 nm due to the formation of nitrophenolate ions.21 Upon the introduction of AuNP− PDA−NDs into the solution, the absorption at 400 nm decreased gradually, along with a concurrent increase in absorption at 300 nm, caused by the formation of 4-AP. In the meantime, the yellow color of the mixture faded to colorless in ∼20 min (Figure 4). This indicated that the 4-NP had been successfully converted to the 4-AP under the catalysis of AgNP−PDA−NDs. The kinetics of the 4-NP reduction process was qualitatively studied by UV−vis spectroscopy. As illustrated in Figure S8, the plot of ln(Ct/C0) versus reaction time (min) showed a good linear correlation with a kinetic reaction rate constant of 0.2492 min−1. Therefore, it can be concluded that this AgNP−PDA−ND-catalyzed 4-NP reduction reaction obeyed the pseudo-first-order reaction kinetics.37−40 Inspired by the above successful reduction of AgNPs on the ND−PDA surface, we then investigated the attachment of thiol-modified DNA (SH-DNA) on the surface of the PDA− NDs because the surface addressability of NDs is extremely important for extending their applications in multidisciplinary areas. It has proved challenging to covalently attach DNA or PNA to the surface of NDs.8,10 Fortunately, the presence of catechol/quinone groups on the PDA layer, which can react with thiol or amine-functional groups through Michael addition and/or Schiff base reactions,21 provides a simple and efficient
Figure 5. Controlled self-assembly of AuNR−ND structures. (A) TEM images of hybridized AuNR−DNA−PDA−NDs via specific DNA hybridization. (B) TEM images of mixture of PDA−ND and AuNRs, which demonstrated the nonspecific physical absorption of AuNRs.
AuNRs attached to the surface of the DNA−PDA−ND nanoconjugates. This suggested that they had bonded together via the sequence-specific hybridization of DNA on both particle surfaces. However, they could also have associated together through nonspecific interactions. Thus, to investigate and determine what had occurred, a control experiment was also carried out by hybridizing the AuNRs with the PDA−NDs treated with corresponding unthiolated DNA through the same conjugation process. The result revealed that there was little (if any) attachment between both nanoparticles (Figures 5B and 4040
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S9). Therefore, these results indicate that DNA could be covalently attached on the surface of PDA−NDs and maintain active biofunctionality. This endowed NDs with addressability and, at the same time, provides the opportunity to incorporate other functional molecules onto the ND surface through DNA hybridization.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b00509. SEM images of uncoated NDs of various sizes; enlarged TEM images of ND (20 and 50 nm) with different PDA thicknesses; TEM images of reduction of AgNPs on the surface of NDs (20 nm) with varied sonication time course and varied concentration of silver solution; SEM images of reduction of AgNPs on the surface of NDs (100 nm) with varied thickness of PDA layer and varied concentration of silver solution; the plot of reduction of 4-NP; SEM images of AuNR−DNA−PDA−ND cluster (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Srikanth Singamaneni: 0000-0002-7203-2613 Risheng Wang: 0000-0001-6539-1565 Notes
The authors declare no competing financial interest. ∥ Y.Z. and W.L. contributed equally to this study.
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REFERENCES
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4. CONCLUSIONS In summary, we have demonstrated a facile and effective surface modification approach for the preparation of ND-based core− shell nanoconjugates using dopamine polymerization. By taking advantage of the universal adhesion and versatile reactivity of PDA, we have effectively conjugated DNA and AgNPs onto NDs. These systematic studies showed that the thickness of the PDA layer and the size and coverage of the AgNPs on the ND surface can be well controlled by varying the PDA concentrations, the [Ag(NH3)2]+ concentrations, and the reaction times. Moreover, the constructed AgNP−PDA−ND and DNA−PDA−ND nanoconjugates were characterized and they exhibited good catalytic activity for 4-nitrophenol reduction and addressability for hybridization with AuNRs, respectively. Combining the unique chemical and physical properties of NDs with the diversity of postreactions of PDA, this approach will dramatically extend the ND’s functionality by immobilization of various species of interests and enhance ND applications in the energy, biomedical, and environmental fields.
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ACKNOWLEDGMENTS
This work is supported by University of Missouri Research Board, Material Research Center, and College of Arts and Science at Missouri University of Science and Technology. Thanks to Dr. Jingwen Ma’s helpful discussion. 4041
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DOI: 10.1021/acs.langmuir.8b00509 Langmuir 2018, 34, 4036−4042
