Nanomechanical characterization of chemical interaction between gold nanoparticles and chemical functional groups
© Lee et al.; licensee Springer. 2012
Received: 15 September 2012
Accepted: 19 October 2012
Published: 31 October 2012
We report on how to quantify the binding affinity between a nanoparticle and chemical functional group using various experimental methods such as cantilever assay, PeakForce quantitative nanomechanical property mapping, and lateral force microscopy. For the immobilization of Au nanoparticles (AuNPs) onto a microscale silicon substrate, we have considered two different chemical functional molecules of amine and catecholamine (here, dopamine was used). It is found that catecholamine-modified surface is more effective for the functionalization of AuNPs onto the surface than the amine-modified surface, which has been shown from our various experiments. The dimensionless parameter (i.e., ratio of binding affinity) introduced in this work from such experiments is useful in quantitatively depicting such binding affinity, indicating that the binding affinity and stability between AuNPs and catecholamine is approximately 1.5 times stronger than that between amine and AuNPs. Our study sheds light on the experiment-based quantitative characterization of the binding affinity between nanomaterial and chemical groups, which will eventually provide an insight into how to effectively design the functional material using chemical groups.
KeywordsAu nanoparticle Dopamine Surface chemistry Atomic force microscopy Lateral force microscopy
Surface chemistry has played a critical role in designing functional nanomaterials for their biological or medical applications such as drug delivery, molecular therapeutics, and diagnostics [1, 2]. In particular, the surface modification of a nanoparticle is of great importance to enhancing functionality in terms of target affinity [3–5], imaging contrast [3, 4, 6, 7], and curative power . For instance, magnetic nanoparticles chemically modified with chemical functional groups or moieties (e.g., ligand and receptor) have been utilized for high-resolution MRI, which is useful in cancer diagnostics since the chemical modification using chemical functional groups or moieties leads to improved targetability and imaging contrasts [3, 6, 7]. Moreover, gold nanoparticles (AuNPs) functionalized with chemical functional groups or moieties have been recently used to enhance photocatalytic performance , to form 3D networks of functionalized AuNPs , and to sensitively detect specific biological molecules (e.g., DNA) [11–13] and cancerous single cells [14, 15].
Dopamine hydrochloride (DOPA) has recently been considered as a chemical linker that allows for efficient surface chemistry useful in not only inorganic materials (e.g., nanoparticles) but also biological materials (e.g., tissue) due to its excellent adhesive property and biocompatibility [16, 17]. In particular, DOPA has been reported as a chemical linker that is useful not only in the chemical modification of the surfaces of nanomaterials such as nanoparticles [18, 19], graphene oxide sheet , and carbon nanotubes , but also in improving binding affinities such as protein-peptide cross-linking , cellular adhesion to substrate , osteoconduction , and hemostatic adhesive in segmentectomy . Despite the broad application of DOPA to surface chemistry using mutual interaction between DOPA and nanomaterials (e.g., nanoparticle), such an interaction has been poorly understood and not yet studied thoroughly. Since the surface modification of nanomaterials using DOPA typically employs a noncovalent conjugation (e.g., coordinate bonding, hydrophobic and electrostatic interactions, etc.) [6, 26], it is essential to establish an experimental framework that allows for measuring a weak binding affinity corresponding to such a noncovalent conjugation, which is useful in the development of drug carrier due to the fact that noncovalent conjugation enables the excretion of waster matter from the human body after the drug carrier completes the function of drug delivery or bioimaging [6, 7, 27, 28].
In this work, we have quantitatively studied a chemical interaction between nanoparticles and chemical functional groups (e.g., DOPA and amine functional group) using experimental toolkits such as cantilever bioassay [29–32], PeakForce Quantitative Nanomechanical Property Mapping (PeakForce QNM) [33, 34], as well as lateral force microscopy (LFM) [35–38]. In a recent decade, cantilever bioassays have been widely utilized for quantitative understanding of molecular interactions on the surface by measuring the bending deflection change [39, 40] and/or shifts in resonance [29, 41]. Moreover, a cantilever has been also employed to measure physical quantities such as temperature , quantum state , and surface stress [29, 44]. We have shown that a cantilever whose surface is functionalized with specific chemical functional groups (DOPA or amine functional group) allows us to quantitatively characterize the binding affinity between nanoparticles and such chemical functional groups. Furthermore, LFM has recently been taken into account for deciphering the molecular interactions by estimating a frictional force that occurs due to breakage of such molecular interactions [35, 38]. In our study, we have employed LFM enabling the movement of a nanoparticle, which is chemically interacting with chemical functional groups on the surface, in order to quantitatively understand the binding affinity between nanoparticle and chemical functional groups by measuring the frictional forces required to break the binding between the nanoparticle and chemical functional groups. In addition, we have also measured the adhesion force between nanoparticles and chemical functional groups using atomic force microscopy (AFM), particularly the PeakForce QNM module. We have shown that the noncovalent interaction between nanoparticles and specific chemical functional groups can be quantitatively studied using the aforementioned experimental techniques (i.e., cantilever assay, PeakForce QNM, and LFM) and that catecholamine (i.e., DOPA) is a chemical functional group useful in the surface modification of nanomaterials (e.g., nanoparticle) due to its excellent binding affinity.
Materials and sample preparation
All materials including gold nanoparticle (G1652, approximately 20 nm in size) and dopamine hydrochloride ((HO)2C6H3CH2NH2·HCl) were purchased from Sigma-Aldrich (St. Louis, MO, USA). A silicon (Si) microcantilever (TESP, Bruker, Madison, WI, USA) was first rinsed by piranha solution (50% of sulfuric acid and 50% of hydrogen peroxide). The cantilever was immersed for 25 min into a 3-aminopropyltrimethoxysilane (APTMS) solution (200 μl/ethanol of 5 ml) for amine functionalization and then carefully washed by ethanol and pure water. The aminated surface of the cantilever (SA) was immersed into the AuNP suspension (approximately 0.01% as HAuCl4) for 30 min for the preparation of AuNP-SA (i.e., AuNP attached to amine-modified surface). In the case of DOPA-functionalized surface (SD), the aminated microcantilever was treated with glutaraldehyde (GA, 10% in phosphate-buffered saline (PBS)) for 30 min for surface activation and then immersed into the DOPA solution (65 mM in PBS at pH 7.4) for 10 h . Consequently, the DOPA-functionalized cantilever was immersed into the AuNP-dissolved solution for the preparation of AuNP-SD (i.e., AuNP bound to DOPA-functionalized surface). All experiment was conducted at room temperature.
Analysis of surface chemistry
Scanning electron microscopy (SEM) imaging was obtained using JSM-6500 F (JEOL, Tokyo, Japan). The number of AuNPs in the SEM images was accurately counted by ImageJ software (NIH, Bethesda, MD, USA). X-ray photoelectron spectroscopy (XPS) analysis was implemented with Escalab 220i-XL (Thermo VG, Hastings, UK). The sampling area was 5 mm × 5 mm in a vacuum of 1.0 × 10−9 mbar with calibration of C 1 s (285 eV). To measure the resonant frequency shift of the cantilever due to AuNP binding onto the cantilever surface, the samples were dried overnight in each fabrication process. The resonant frequency of the cantilever is measured using the Nanoscope V controller (Veeco, Santa Barbara, CA, USA).
Measurement of adhesion/friction forces
PeakForce QNM was used to measure the adhesion between AuNPs and chemically functionalized surface using the BioScope Catalyst (Veeco). For PeakForce QNM imaging, we have used a cantilever, particularly ScanAsyst Air probes (kN = 0.58 N/m; Bruker) in 22.2°C and 38% humidity. For LFM imaging, we have employed various AFM cantilever tips (i.e., SNL-10, ScanAsyst Air, ScanAsyst Fluid, Bruker) with their stiffness in the range of 0.1 to 1 N/m. LFM images were obtained by scanning the sample in contact mode with a scan size of 2 × 2 μm2, scan rate of 0.5 Hz, and a set point of 1 V. The detached AuNPs from the surface was confirmed by using PeakForce QNM imaging. All AFM, LFM, and PeakForce QNM images were analyzed with NanoScope Analysis software (Bruker).
Results and discussion
Characterization of the AuNPs and surface chemistry of SAand SD
Indirect measurement of the binding affinity between AuNPs and chemical functional groups
The resonant frequency shift of AFM cantilevers in cantilever assay
300.2 ± 32.3
299.8 ± 31.7
298.4 ± 32.6
279.8 ± 3.5
279.5 ± 3.8
276.9 ± 4.0
274.2 ± 4.0
Direct measurement of the binding affinity between AuNPs and chemical functional groups
Summary of triangular microcantilever parameters (SNL and ScanAsyst Fluid) used in LFM study
Here, Slat is the lateral sensitivity of the cantilever defined as Slat = PHSnorm/aR*L, where P is a proportionality factor (≈2.5 for the triangular cantilever), Snorm is the vertical deflection sensitivity of the cantilever, a is the amplification factor of the lateral signal measured, and R* is the ratio of the beam height to the beam width (R* = 0.5) . ΔV is the measured value in LFM analysis, which is extracted from the LFM images. In general, the longer and larger the cantilever, the lower is its normal spring constant (i.e., more flexible in normal deflection), but the larger is its lateral spring constant (klat). We can control the exerted applied force using different spring constants of cantilevers (knorm) under an identical deflection set point (1 V) rather than a set point control with an identical AFM tip in order to avoid damage to the samples and a subsidiary frictional noise.
In conclusion, we have demonstrated a quantitative characterization of the binding affinity between AuNPs and chemically modified surface using various experimental techniques such as SEM image analysis, cantilever assay, PeakForce QNM, and LFM image analysis. It is shown that the DOPA-modified surface is an effective conjugation method for functionalization of nanoparticles onto the surface when compared with amine-modified surface, as anticipated, from our various experiments. More remarkably, we have shown that dimensionless parameters (i.e., RN, RM, and RF) introduced in this work are useful in quantifying the binding affinity between nanoparticle and chemical functional groups, and that these dimensionless parameters are consistent regardless of experiments, i.e., RN, RM, and RF are almost identical to each other, implying that the binding affinity between nanostructure and chemical group can be quantitatively studied using either indirect method (i.e., SEM image analysis and cantilever assay) or direct method (i.e., lateral force measurement). Our study sheds light on how to quantitatively study the binding affinity between nanostructure and chemical functional group, which can provide the design principles for nanoparticle-based systems such as nanomedicine and nanobiosensor.
Atomic force microscopy
- PeakForce QNM:
PeakForce quantitative nanomechanical property mapping
Lateral force microscopy
- S A :
- S D :
Scanning electron microscope
X-ray photoelectron spectroscopy.
This work is supported by the National Research Foundation (NRF) of Korea (under grant nos. NRF-2010-0009428, 2010–0027238, 2011–0009885, and 2012R1A2A2A04047240).
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