Gold Nanoparticle Analysis

Gold nanoparticles, sometimes known as colloidal gold, have distinctive properties that are advantageous to many applications. Usually, they are produced by controlled reduction of aqueous HAuCl4 solution using a reducing agent such as citrate, under changing conditions.

Several kinds of gold nanoparticles are available depending on the shape, size, and physical properties, as represented in Figure 1. Applications of gold nanoparticles comprise of drug delivery (1), carriers for drugs like Paclitaxel (2), tumor detection (3), biosensors, and many more.

Types of gold nanoparticles (stylized)

Figure 1. Types of gold nanoparticles (stylized)

The size of gold nanoparticles is a vital physical parameter (4) needing cautious measurement. The particle size influences properties like surface plasmon resonance (SPR) peak, absorbance wavelength (increased size = longer wavelength), blood half-life, intracellular uptake, and bio-distribution profile (decreasing size = increased blood half-life).

The particle size and width can be used as an indicator for suspension stability. In addition, surface charge (zeta potential) measurements are used as an indicator for suspension stability.

Dynamic light scattering (DLS) and electrophoretic light scattering (ELS) are the most accepted method for particle size and zeta potential analysis. The Nicomp Z3000 system (Figure 2) is appropriate for determining the zeta potential and size of gold nanoparticles.

Nicomp Z300 system

Figure 2. Nicomp Z300 system

Materials

The Nicomp DLS system was used to analyze the size and zeta potential of many gold nanoparticle samples. Three different gold nanoparticle samples were bought from Sigma Aldrich with nominal sizes of 5, 20, and 50 nm. Additionally, NIST reference material 8012 with a nominal size of 30 nm was examined. The reference physical properties of these samples are given below. Size analysis outcomes by DLS are represented as the intensity mean of the hydrodynamic diameter.

The width of the distribution is represented as the polydispersity index (PDI). The NIST 8012 Report of Investigation offers mean hydrodynamic diameter at two angles, backscatter at 90° and 173°. In contrast to the samples obtained from Sigma Aldrich, the reference zeta potential value for NIST 8012 is also given.

Sample Physical Properties

Sigma Aldrich P/N 742007

  • Core size: 47–53 nm
  • Mean hydrodynamic diameter (Z): 58–66 nm
  • Polydispersity index (PDI): <0.2

Sigma Aldrich P/N 741965

  • Core size 18–22 nm
  • Mean hydrodynamic diameter (Z): 28–36 nm
  • Polydispersity index (PDI): <0.2

Sigma Aldrich P/N 741949

  • Core size: 4–7 nm
  • Mean hydrodynamic diameter (Z): 14–25 nm
  • Polydispersity index (PDI): <0.2

NIST RM 8012

  • DLS, 90°: 26.5 ± 3.6 nm
  • DLS, 173°: 28.6 ± 0.9 nm
  • Zeta potential: −33.6 ± 6.9 mV
  • Polydispersity index not given

Method

A Nicomp Model Z3000 (Figure 2) was used to analyze all the samples for particle size and zeta potential. Before these studies began both size and zeta potential standards were analyzed to check system performance. A concentration study was carried out on sample 742007 to examine if reported particle size changed with concentration. The outcomes from this research denoted that the results were insensitive to concentration, and therefore all measurements were made at the original sample concentration without dilution. A time-of-analysis study was carried out on sample 741949 to determine an ideal measurement duration based on obtaining stability as demonstrated in the Time History plot in Figure 3. This work showed that an analysis time of 5 minutes was adequate.

Time history plot Diameter (nm) vs Time (minutes)

Figure 3. Time history plot Diameter (nm) vs Time (minutes)

Figures 4 and 5 illustrate the Nicomp size and zeta potential analysis settings for all measurements.

Nicomp size settings

Figure 4. Nicomp size settings

Nicomp zeta potential settings

Figure 5. Nicomp zeta potential settings

Results — Particle Size

Figures 6 to 10 represent typical graphical results for two analyses for each sample.

50 nm Au overlay of two size results

Figure 6. 50 nm Au overlay of two size results

20 nm Au overlay of two size results

Figure 7. 20 nm Au overlay of two size results

Sample 741949 (nominal size 5 nm) posed further problems in producing results approximate to the predicted values. Figure 8 illustrates the Gaussian result for the sample when analyzed without any extra sample preparation. Gaussian calculations drive the result into one single peak.

5 nm Au overlay of two size Gaussian results

Figure 8. 5 nm Au overlay of two size Gaussian results

Furthermore, the Nicomp Z3000 system can yield multi-modal results with the help of the proprietary Nicomp algorithm. The result illustrated in Figure 8 was recalculated using the Nicomp algorithm. Figure 9 shows the resultant multi-modal size distribution.

5 nm Au overlay of two size Nicomp results

Figure 9. 5 nm Au overlay of two size Nicomp results

Subsequently, the sample tube was centrifuged for 8 minutes in an effort to separate the larger particles from the measurement zone. The same sample tube was analyzed following centrifugation and the result is demonstrated in Figure 10.

5 nm Au overlay of two size results, centrifuged

Figure 10. 5 nm Au overlay of two size results, centrifuged

Sample NIST 8012 was examined at three angles — backscatter at 170° (pink), 90° (green), and the forward angle at 15° (blue). These three results are illustrated in Figure 11.

NIST 8012 Au overlay of three size results,170°, 90°, and 15°

Figure 11. NIST 8012 Au overlay of three size results,170°, 90°, and 15°

Table 1 summarizes these results plus zeta potential values.

Table 1. Expected and reported size and zeta potential results

Sample Expected Result Reported Size (nm) Zeta Potential (mV)
742007 58-66 64 -34.58
741965 28-36 34 -27.95
741949 14-25 15.6 -42.43
8012, 170° 28.6 ± 0.9 31.9
8012, 90° 26.5 ± 3.6 30.8
8012, 15° 66.6
8012 zeta -33.6 ± 6.9 -30.49

50 nm Au zeta potential results

Figure 12. 50 nm Au zeta potential results

20 nm Au zeta potential results

Figure 13. 20 nm Au zeta potential results

5 nm Au zeta potential results

Figure 14. 5 nm Au zeta potential results

NIST 8012 Au zeta potential results

Figure 15. NIST 8012 Au zeta potential results

Discussion

The results for Sigma Aldrich 50 and 20 nm nominal size samples were within the projected value range. The Sigma Aldrich 5 nm nominal size results initially reported particle size significantly higher than expected – indicating aggregation. This was not unexpected given the color of this sample as illustrated in Figure 16, far left. A darker, bluer color usually denotes a much larger particle size distribution. The result prior to centrifugation (Figure 9) shows the presence of primary particles near 14 nm and aggregate peaks at 122 and 417 nm. This result exhibits the power of the Nicomp algorithm to resolve primary particles as well as multiple peaks of aggregates. Following centrifugation, the reported mean size was within the predicted value range.

Left to right: 5, 20, and 50 nm Au sample bottles

Figure 16. Left to right: 5, 20, and 50 nm Au sample bottles

The NIST 8012 sample was examined at three different angles: 15°, 90°, and 170°. The faintly smaller result at 90° vs 170° implies that 90° is a better measurement condition versus backscatter — consistent with other earlier reported results (5). Measurement in the forward angle direction (15°) enhanced sensitivity to larger particles, as illustrated in Figure 11. These results imply that a multi-angle system (a standard and inexpensive Nicomp option) may be better for some particular applications.

The zeta potential results were all tremendously repeatable using the PALS method. In comparison with disposable zeta potential cells, the Nicomp dip cell can measure thousands of samples with a lower cost of ownership.

Stability Study

A stability study was carried out on the NIST 8012 sample to examine the impact of salt concentration on appearance and size. The original size result is demonstrated in Figure 11 with a reported size of 30.8 nm at 90°. First, 200 µL of NIST 8102 was pipetted into 200 mL filtered DI water. Then, KCl at 3 M was added to the diluted NIST 8012 suspension in increments of 500 µL while examining the color of the sample. A color change was observed following the addition of 2 mL of 3 M KCl. Figure 17 illustrates, from left to right, the NIST 8012 sample after combining the KCl, original sample in the cell, and the original sample bottle.

After KCL added, before, original bottle

Figure 17. After KCL added, before, original bottle

Figure 18 illustrates the particle size distribution of the NIST 8012 after adding KCl. Over the 15 minute analysis period, the size increased as a function of time. After 24 hours, the sample had fully aggregated and had deposited at the bottom of the cuvette as demonstrated in Figure 19.

NIST 8012 Au overlay of three size results after KCl addition

Figure 18. NIST 8012 Au overlay of three size results after KCl addition

NIST 8012 with settled aggregate

Figure 19. NIST 8012 with settled aggregates

Conclusions

Dynamic light scattering is the ideal technique for zeta potential and particle size analysis of nano-gold particles. The Nicomp Z3000 has been established as a high-resolution, accurate method yielding results near to the projected values. The multi-modal mix of primary and aggregated particles for the aggregated samples was detected using the Nicomp algorithm.

References

  1. Han G, Ghosh P, Rotello VM (February 2007). “Functionalized gold nanoparticles for drug delivery.Nanomedicine. 2 (1): 113–23.
  2. Gibson JD, Khanal BP, Zubarev ER (September 2007). “Paclitaxel-functionalized gold nanoparticles.Journal of the American Chemical Society. 129 (37): 11653–61.
  3. Sajjadi AY, Suratkar AA, Mitra KK, Grace MS (2012). “Short-Pulse Laser-Based System for Detection of Tumors: Administration of Gold Nanoparticles Enhances Contrast.J. Nanotechnol. Eng. Med. 3 (2): 021002.
  4. Dreaden EC, Austin LA, Mackey MA, El-Sayed MA. Size matters: gold nanoparticles in targeted cancer drug delivery. Ther Deliv. 2012;3(4):457–78.
  5. PSS Technical Note 724—Multiangle DLS Measurements

This information has been sourced, reviewed and adapted from materials provided by Particle Sizing Systems, An Entegris Company

For more information on this source, please visit Particle Sizing Systems, An Entegris Company

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