Gold Nanoparticles: Worth their Weight in Gold!

Recent discussion in our household on merit of buying gold jewelry somehow led my thoughts to their application in Biology and hence this post. My first exposure to gold for biological application came when I started making self assembled monolayers (SAMs) on Gold surfaces. However, very soon my research life took a detour when working with Prof. Chilkoti we demonstrated for the first time the use of gold colloids/nanoparticles for label free bio-sensing. Things change, and now I have only a tangential link to gold in the form of Biacore SPR chips for biomolecular interaction studies. So I decided to take stock of all the current applications of Gold nanoparticles in biology especially in Biosensing.

Gold nanoparticles display unique size and shape dependent optical properties. An aqueous solution of gold particles of 20 nm diameter appears brilliant red but the color changes to purple as the nanoparticle size increases or shape changes. Nanoparticle color also changes to purple as the gold particles aggregate. The colors exhibited by gold nanoparticles arise from interaction of incident light with the “free” electrons resulting in resonant excitation of an oscillating dipole (surface Plasmon resonance). The color change is only one of the several modalities that have been exploited for design of biosensors. Some applications of gold nanoparticles are:

  1. Gold Nanoparticles for Electron Microscopy: Gold nanoparticle labeled with specific antibodies are used to stain tissues, cells etc that are then imaged using TEM. This is one of the earliest and most extensively used applications of gold nanoparticles.

     

  2. Gold nanoparticle based dip-stick assays OR Lateral-flow Immunoassay: Dip-stick or lateral-flow immunoassays are perhaps the most commercially successful application of gold nanoparticles.

     

     

    Principle is simple. Membrane test strip contains gold nanoparticles conjugated with antibody (G-Ab) against specific analyte (Pregnancy strip has antibody specific for hCG). When a drop of urine is added, the analyte binds to G-Ab and the complex move chromatographically along the membrane. An immobilized capture antibody then binds the complex and concentrates the gold nanoparticles that then appear as red line. Pregnancy tests and drugs-of-abuse tests are the most commonly used application because urine samples provide optimum condition for stability and affinity of antibodies. Complex matrices like blood or saliva contain high amount of proteins, lipids, and polysaccharides and may need preprocessing of samples which may be integrated into the test strips.

     

  3. Gold nanoparticles for Surface Enhaced Raman Spectroscopy (SERS) Applications (1,2): Raman spectroscopy detects and identifies molecules based on their vibrational energy. Typically, Raman scattering is weak that limits the sensitivity of this technique. However, when molecules are placed on or close to the gold nanoparticles the signal from Raman scattering can increase by a factor of 105-1015. The increase is thought to be due to the interaction between surface plasmon of gold nanoparticles and incident/scattered light from Raman active molecule. Signal enhancement depends on the shape and size of the nanoparticles. Silver coated gold nanoparticles may further enhance the sensitivity of the method. Single molecule detection is possible using SERS and several protein and nucleic acid biosensors have been designed using this principle.

     

  4. Gold Nanoparticles for Distance Dependent Detection of DNA: This method leverag the distance dependent optical properties of gold nanoparticles. Chad Mirkin in a very simple but elegant way for the first time used this approach for detection of DNA hybridization.

     

     

     

    Since this first demonstration, several other bio-recognition events have been detected by leveraging the distance dependent color change in gold nanoparticles. Few examples are a) Streptavidin-Biotin; b) Antibody-antigen; c) Lectin-sugar; d) Aptamer-analytes; e) protein-protein interactions; etc. Turning the events around-any biological event that can reverse the aggregation can also be detected using gold nanoparticles. Proteases, phosphatase, and β-lactamase have been detected using this approach.

     

  5. Gold nanoparticle with silver enhancement: Silver enhancement refers to electroless silver deposition where gold nanoparticle act as catalysts to reduce silver ions (I) to metallic silver in the presence of a reducing agent (such as hydroquinone). The metallic silver is quantitated by electrochemical measurements, colorimetric methods etc. Immunoassays as well as DNA hybridization can be very sensitively detected using this method.

     

     

  6. Gold nanoparticles for nanoSPR based detection: Gold nanopartcicles also change color from red to purple in response to refractive index change in the vicinity of nanoparticle surface. Antibodies can be easily attached to the nanoparticles and when the specific analytes bind to the antibodies, color changes in proportion to analyte concentration.

     

     

    This label free sensing is easy to implement in several formats.

    1. Solution based sensing: Gold nanoparticles in solution can be functionalized with antibodies and color change in presence of analyte can be detected in a regular spectrophotometer using a cuvette. Problem with this approach is that activating nanoparticles with proteins becomes a multiple step process involving tedious washing using high speed centrifugation. Multiple washing steps lead to loss of nanoparticles and may require normalization. A bigger problem is the detection of analytes in biological samples like blood or serum. Scattering from complex samples can cause ambiguous results
    2. Nanoparticle chip based sensing: A simpler approach is to make gold nanoparticle biochips. Gold has high affinity for amine and thiol groups and will make a self assembled monoloayer on glass surfaces activated with silanes containing amine and thiols. Biochips can be handled easily for activation with biomolecules for analyte recognition. Biochips can be read easily in a spectrophotometer (after some tweaking to accommodate chips). Complex samples are also easy to handle by using flow system and end point assay format. Sensitivity of the bioassay can be modulated by changing the shape and size of the nano-particles.
    3. Nanostructure chip based sensing: Instead of using gold nanoparticles synthesized from solution chemistry, gold nanostructures can be created directly on glass substrate using techniques like nanosphere lithography and e-beam lithography. The particles shape, size and positioning can be tuned to maximize the sensitivity of the biochip. An additional advantage over solution based gold nanoparticle may be the reproducibility because of exact control of nanoparticle shape and positioning. An obvious disadvantage is the cost.

       

  7. Gold nanoparticle sensing using scattering properties: Due to surface plasmons, the scattering of visible light is so intense that gold nanoparticles can be easity seen with a naked eye or using a regular microscopy by attaching dark field attachment. Yguerabide et.al. (1, 2) calculated that a 52nm gold nanoparticle has a scattering cross-section in microns and the scattered light intensity of a single particle is equal to ~105 fluorescein molecules. As the refractive index of medium surrounding the gold nanoparticles change the scattering particle intensity increases and the color of the scattered light shifts to longer wavelengths. This phenomenon has been used for very sensitive biosensing.

     

  8. Gold and Silver Nanobarcodes: Multiplexed biosensing can be done using optically distinguishable metallic nanowires made of gold and silver segments. Encoded metal nanoparticles of gold and silver segments are synthesized by templated electrodeposition or using on-wire lithography (OWL) and have been used for multiplexed protein or nucleic acid detection.

     

In addition to gold nanoparticles, gold thin films are being extensively used for designing biochips for Surface Plasmon Resonance measurements and for making self assembled monolayers (SAMs). Will cover these applications later. So to finish, Gold for biological applications is indeed worth its weight in gold!

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4 responses to “Gold Nanoparticles: Worth their Weight in Gold!

  1. Pingback: Gold Nanoparticles for Cancer Cell Detection « All About Biosensors

  2. Pingback: Self Assembled Monolayers (SAMs) on Gold: Celebrating 25 (or so) years! « All About Biosensors

  3. Pingback: NanoPen for Patterning Nanoparticles « All About Biosensors

  4. Pingback: Getting a Handle on Nanostructures for Better Biosensors « All About Biosensors

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