Conjugation of Proteins to NHS-Activated Gold Nanoparticles

1. Introduction

The conjugation of proteins to NHS-activated gold nanoparticles is a widely used technique in various fields, such as biosensing, drug delivery, and imaging. This protocol provides a detailed step-by-step guide for achieving efficient and stable conjugation. NHS (N-Hydroxysuccinimide)-activated gold nanoparticles have reactive ester groups on their surface, which can react with primary amine groups present in proteins, typically from lysine residues, to form stable amide bonds. This covalent conjugation offers several advantages over passive adsorption methods, including enhanced stability of the conjugate and reduced risk of protein desorption over time.

2. Combining principles

The role of NHS: NHS (N-hydroxysuccinimide) is often used in combination with 1- (3-dimethylaminopropyl) -3-ethylcarbodiimide (EDC) to activate carboxyl and other functional groups on the surface of gold nanoparticles. EDC first activates the carboxyl group to form an unstable intermediate, and then NHS reacts with the intermediate to generate a stable NHS ester. This NHS ester has high reactivity and can react with amino groups on protein molecules, thereby achieving covalent binding between proteins and gold nanoparticles. The advantages of covalent binding: Through this covalent binding method, proteins can be stably attached to the surface of gold nanoparticles. Compared with noncovalent interactions (such as electrostatic interactions, hydrophobic interactions, etc.), covalent binding can effectively prevent proteins from falling off the surface of nanoparticles during subsequent use, ensuring the stability of the complex and facilitating long-term effects in vivo or in vitro environments. Factors affecting the combination: Buffer-related factors: The type, pH value, and concentration of the buffer have a significant impact on the binding of proteins to NHS-activated gold nanoparticles. Different buffer solutions (such as HEPES, Tris HCl, boric acid, PBS buffer, etc.) can affect the reaction microenvironment due to their different chemical properties.

3. Influencing factors:

a. Protein properties: The type, concentration, and isoelectric point of the protein can affect its binding with gold nanoparticles.

 For example, the amino acid composition and spatial structure of different proteins vary, resulting in differences in the number and distribution of amino groups available for binding on their surfaces.

b. Charge: The charge distribution on the protein surface determines the electrostatic interaction between it and gold nanoparticles with opposite charges

c. Protein concentration: Protein concentration affects the probability of binding with gold nanoparticles.

 Within a certain range, the higher the protein concentration, the greater the chance of collision and binding with gold nanoparticles, and the binding amount may increase.

Properties of gold nanoparticles

d. Particle size: The particle size of gold nanoparticles affects their specific surface area and surface energy, which in turn affects their binding to proteins. Smaller-sized gold nanoparticles have a larger specific surface area and can provide more binding sites, which may lead to tighter binding with proteins; Larger-sized gold nanoparticles may affect binding in terms of steric hindrance and other aspects. When studying the interaction between gold nanoparticles of different sizes and proteins, it was shown that the interaction between gold nanoparticles of different sizes and proteins is different. This can be analogized to the binding of activated gold nanoparticles with NHS, indicating that particle size is one of the influencing factors.

e. Surface modification: The surface modification of NHS-activated gold nanoparticles determines their chemical properties and binding ability with proteins. NHS (N-hydroxysuccinimide) groups can react with amino and other groups in proteins to form stable amide bonds. If the surface of gold nanoparticles is modified with other functional groups, such as PEG (polyethylene glycol), it will change their hydrophilicity, hydrophobicity, steric hindrance, etc., affecting the binding between proteins and gold nanoparticles. As mentioned in the literature, modifying gold nanoparticles with different ligands affects their interaction with proteins, indicating the importance of surface modification for binding

4. Purification of Conjugated Nanoparticles

1. Centrifuge

Transfer the quenched reaction mixture to a microcentrifuge tube and centrifuge at high speed for 10-15 minutes. Conjugated nanoparticles will precipitate at the bottom of the test tube, while unreacted proteins, buffer components, and any by-products will remain in the supernatant. Carefully remove the supernatant using a pipette, being careful not to interfere with the nanoparticle particles.

2. Washing

Add an appropriate amount of washing buffer to the nanoparticles.

 The volume of the washing buffer can be similar to the volume of the original reaction mixture, gently moving the liquid up and down or briefly vortexing to resuspend the nanoparticles.

Tween-20 in washing buffer helps prevent nanoparticle aggregation during the washing process. Centrifuge the resuspended nanoparticles again at the same high speed for 10-15 minutes. As mentioned earlier, remove the supernatant. Repeat the washing steps 2-3 times to ensure complete removal of any unreacted substances¡£

3. Resuspend

After the final wash, resuspend the purified conjugated nanoparticles in an appropriate volume of PBS. The volume of resuspension depends on the concentration of conjugated nanoparticles required for subsequent applications.

5. Characterization of Conjugated Nanoparticles

1. Visual Inspection

Observe the color of the conjugated nanoparticle suspension. A change in color compared to the original NHS-activated gold nanoparticles can indicate successful conjugation. For example, if the original nanoparticles were a deep red color and the conjugated nanoparticles appear a slightly different shade, it may suggest that the protein has bound to the nanoparticles and altered their optical properties.

Check for any signs of aggregation, such as the presence of visible clumps or a cloudy appearance in the suspension. Aggregation can affect the performance of the conjugated nanoparticles in subsequent applications.

2. UV-Vis Spectroscopy

Measure the UV-Vis absorption spectrum of the conjugated nanoparticles using a spectrophotometer. Compare the spectrum with that of the original NHS-activated gold nanoparticles. The absorption peak of gold nanoparticles typically shifts upon conjugation with proteins due to changes in the local environment around the nanoparticles. The magnitude and direction of the shift can provide information about the extent of conjugation and the conformation of the protein on the nanoparticle surface.

Calculate the ratio of the absorbance at the characteristic wavelength of the protein (if it has a distinct absorbance, e.g., at 280 nm for many proteins due to tryptophan, tyrosine, and cysteine residues) to the absorbance at the surface plasmon resonance wavelength of the gold nanoparticles. This ratio can be used to estimate the amount of protein conjugated per nanoparticle.

3. Dynamic Light Scattering (DLS)

Use DLS to measure the hydrodynamic size of the conjugated nanoparticles. The hydrodynamic size is expected to increase compared to the original nanoparticles due to the addition of the protein layer. The increase in size can be used to assess the successful conjugation and to estimate the thickness of the protein layer on the nanoparticle surface.

Analyze the size distribution obtained from DLS. A narrow size distribution indicates that the conjugated nanoparticles are relatively homogeneous, while a broad distribution may suggest the presence of aggregates or a heterogeneous population of conj. 

Transmission Electron Microscopy (TEM): TEM is a powerful tool for directly visualizing the size, shape, and internal structure of conjugated nanoparticles.ugated nanoparticles.

Scanning Electron Microscopy (SEM): SEM images can show whether the nanoparticles are aggregated or well-dispersed, which is important for understanding their stability and performance in applications. Aggregated nanoparticles may have different properties compared to single, well-dispersed particles.

6. Application

1. Targeted tumor imaging

2. Protein detection

Fluorescence detection, based on the detection of streptavidin using small molecule linked DNA end protective junction alloy nanoparticles fluorescent probes, can utilize NHS-activated gold nanoparticles to better bind with specific DNA probes

3. Disease diagnosis: Abnormal expression of certain special proteins is closely related to diseases such as tumors and cancers.

 By utilizing the specific binding properties of NHS-activated gold nanoparticles to these proteins, a detection system can be constructed for the early diagnosis of diseases.

4. Drug delivery: Due to protein binding, it increases drug targeting and optimizes the effectiveness of drug delivery.

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