Background
Colloidal gold nanoparticles have been used to stain glass windows 100s of years ago, before they even knew they were working with AuNPs.
AuNPs are used for heating on-demand for drug delivery applications which can expedite the release, for heating in a killing sense for cancer, for (bio-)sensors using aggregation techniques, in electronic conductors, DNA detection, and much more. The optical/electronic properties of AuNPs can be easily modified using the size, shape, aggregation state (in solution or not), or the surface chemistry.
DIAGNOSTIC APPLICATIONS
Gold nanopaticles are very easily conjugated to antibodies and other proteins due to the affinity of sulfhydyl (-SH) groups for the gold surface, and gold-biomolecule conjugates have been widely incorporated into diagnostic applications, where their bright red color is used in home and point-of-care tests such as lateral flow assays.
BIOMEDICAL APPLICATIONS:
Gold nanomaterials can be easily conjugated to biomolecules to specifically target cancer cells, and used for photothermal cancer therapy, where their tunable optical properties cause them to convert laser light into heat and selectively kill cancerous cells.
Citrate-stabilized gold nanoparticles
This is a useful link for learning about citrate-stabilized gold nanoparticles: https://nanocomposix.com/pages/citrate-surface
Citrate-stabilized gold nanoparticles are the easiest gold nanoparticles to synthesize, in my opinion. You can simply create a 0.01% solution of tetrachloroauric acid trihydrate (which is 0.1 mg/mL of solution). You can then boil this solution vigorously in a reflux condenser. The purpose of the reflux condenser is so that you don’t lose any of the solution as you are boiling it. The solution simply recondenses and falls back into the solution. Boil this solution for a few minutes so that it reaches a steady state in terms of the amount at the bottom of the round bottom flask and what is in the reflux condenser. You can then inject the sodium citrate in a 1% solution or 10 mg/mL solution into the already boiling 0.1% solution of tetrachloroauric acid. It is important to inject the sodium citrate rapidly. The faster, the more homogeneous the size distribution of the gold nanoparticles will be. You will want to inject the sodium citrate just above the boiling solution. I liked to get a long tube that my solution would go through so that it injected directly into the tetrachloroauric acid solution. If you don’t do this, the gold nanoparticle synthesis procedure will not be very repeatable. This is because when you inject the sodium citrate solution, some will get captured along the walls of the reflux condenser and not all of it will end up at the same time in the tetrachloroauric acid solution.
Some practical tips I have learned is that when you purchase the tetrachloroauric acid, you will receive some chunks in a glass vial that is not in solution. It is dry. However, if you leave the cap open for long, it will turn into a liquid because of the humidity in the air. This will screw up the mass measurements. I would highly recommend measuring all of the mass immediately in individual vials where you write the mass of the initial measurements. When you create a 0.1% solution, the hydrated mass differences will be negligible, however.
Another very important thing to remember when dealing with tetrachloroauric acid solutions is that, the solution can be reduced with metals. For example, a lot of the borosilicate glass vials that are in labs have metal inserts at the top where the lids are to help seal liquid within when closed. If your tetrachloroauric acid solution touches this metal lining you will end up an unknown concentration of the solution, in addition to black particulates in solution. This is highly undesirable, of course.
When you inject the sodium citrate solution when synthesizing, you can do this in different ratios to control the size. The less sodium citrate you add, the larger the gold nanoparticles will end up. A good range of sizes that you can expect with this method is from 10 nm – 100 nm. If you are wanting diameters outside this range, I’d recommend a different synthesis method.
These AuNPs are citrate-stabilized, meaning they have carboxylic acid groups on the AuNPs, making them highly negatively charged or anionic – typically 10s of mVs in magnitude. If you imagine having a lot of entities in solution that are all negatively charged, they want to stay away from each other which keeps them in solution. If they were neutral they would all crash out of solution. If they were all positively charged, that would also keep them in solution. The carboxylic acid groups are highly convenient though because you can do conjugation to other entities using the carboxylic acid groups (i.e., with 11-mercaptoundecanoic acid – refer below). Note that these are citrate stabilized in a coated sense and not in a conjugated sense. Meaning, if you centrifuge these AuNPs, i.e., at 10 krpm for 10 min, and was aspirate 95% of the volume, add water, and repeat a couple of times, the AuNPs will all crash out. However, if you aspirate 95% of the volume but add 10 mg/mL of the sodium citrate again, they will not crash out of solution. You can sonicate them back into solution as well if they did crash out.
Characterization of AuNPs
AuNPs are regularly characterized by measuring the absorbance over a spectrum of wavelengths. Usually this instrument is a referred to as a spectrophotometer or a UV-Vis spectrophotometer. Generally the wavelengths range from 200 nm or so up to 800 nm. If you are going to work with AuNRs, you will want a spetrophotometer that can reach the higher end of the spectrum or the near-infrared (NIR) region. When you measure the absorbance versus wavelengths, there are a few key characteristics that you want to note: the peak of the absorbance, which is the surface plasmon resonance (SPR) wavelength, the absorbance value/cm, and the width of the peak (this can be easily done using the full width-half maximum value (this is less commonly used, however). I’d like to point out that in many cases the absorbance values reported from such an instrument are already normalized to cm. This is important because the pathlength the light travels through will be different based on a number of factors, such as volumes. If the absorbance value is already normalized then it is far easier because you can more easily work with Beer-Lambert’s law (Absorbance=epsilon*Concentration*length); epsilon is absorbance*M^-1. If you know the extinction coefficient for an entity, then you can easily calculate the concentration just be measuring the Absorbance. What is generally reported is Absorbance/length=epsilon*concentration. A good publication for this is: https://pubs.acs.org/doi/10.1021/ac0702084
For the citrate-stabilized AuNPs, you will find that the SPR wavelengths range from 510 nm generally and upwards. The 10 nm in diameter AuNPs will be at the 510 nm range and the 100 nm will be quite a bit higher. The relationship is exponential based on my experience for the diameters between 10 nm and 100 nm. Note that there is a red-shift in the wavelength as the size of the AuNPs increase. This means that if the AuNPs increase, they will absorb more red light than before, relatively speaking. Note, that if all of the AuNPs aggregate, they will fall out of solution and the absorbance drops effectively to zero (however the black particulates can result in absorbance depending on where it sits in the plate being measured in the spectrophotometer; you can have some higher variability of absorbance and much lower values).
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UV-Vis Tutorial | Part 3: Data Analysis
Adding salt to charged AuNPs.
Imagine having carboxylic acid coating AuNPs. If you add salt, the cations will then ionically complex with the AuNPs causing them to be less charged and therefore result in more aggregation, and a red-shift in the surface plasmon resonance (SPR) wavelength.
Adding acid to citrate-stabilized AuNPs
If you add acid to carboxylic acid or citrate-stabilized AuNPs, then the carboxylic acid groups become protonated and this also causes the AuNPs to become less charged, leading to aggregation, and a red-shift in the SPR wavelength.
Characterizing AuNPs less than 5 nm
These AuNPs are brown. Interestingly, AuNPs less than about 5 nm or so, the SPR wavelengths jump from 510 nm down to the UV range. If you are characterizing these types of AuNPs then you will want to ensure you are using acrylic plates when measuring absorbance and not the more common polystyrene plates. This is because polystyrene absorbs in the same range and will confound the absorbance of the AuNPs.
Synthesizing <5 nanometer (nm) AuNPs
When I think of synthesizing AuNPs that are <5 nm, I think of ice and a really strong reducing agent. Sodium citrate is used for reducing the tetrachloroauric acid, or in other words giving electrons to tetrachloroauric acid. However, when synthesizing extremely small AuNPs, we need an even stronger acid or stronger reducing agent, such as sodium borohydrate. You need to be careful with all of the reagents you work with but when you are mixing your solution used for synthesizing AuNPs, take extra care to not inhale the powder inadvertently.
AuNPs in organic solvents
Coming.
Gold Nanorods (AuNR)s
Here is a PDF from nanoComposix/NanoXact(TM) to conjugate carboxyl groups to the gold nanorods. After the carboxyl groups are on the gold nanorod, you can then do further interesting conjugations to the gold nanorods.
Branched AuNPs
Coming.
Gold Nanoshells
Coming.
Uses of AuNPs
Coming.
Coating AuNPs with polyelectrolytes (i.e., charged polymers)
Coming.
Citrate-stabilized Gold Nanoparticles
| Source | Product | Price* | Webpage |
|---|---|---|---|
| bbisolutions.com | Diagnostic Gold Colloid starter pack – 20nm / 40nm / 60nm / 80nm | $251.60 | LINK | bbisolutions.com | Diagnostic Gold Colloid Starter Pack PLUS – 20nm / 40nm / 60nm / 80nm | $750.30 | LINK |
Synthesis video tutorials
Plasmonics Nanoparticles
Gold nanoparticles have unique optical properties because they support surface plasmons. At specific wavelengths of light the surface plasmons are driven into resonance and strongly absorb or scatter incident light. This effect is so strong that it allows for individual nanoparticles as small as 30 nm in diameter to be imaged using a conventional dark field microscope. This strong coupling of metal nanostructures with light is the basis for the new field of plasmonics. Applications of plasmonic gold nanoparticles include biomedical labels, sensors, and detectors. The gold plasmon resonance is also the basis for enhanced spectroscopy techniques such as Surface Enhanced Raman Spectroscopy (SERS) and Surface Enhanced Fluoressence Spectroscopy which can be used to detect analytes with ultrahigh sensitivity.
QC Excel Template for UV-Vis Analysis: Link
Storage & Handling Information