‘Nano-impacts’: An Electrochemical Technique for Nanoparticle Sizing in Optically Opaque Solutions

Typical laser-dependent methods such as nanoparticle tracking analysis (NTA) and dynamic light scattering (DLS) are not able to detect nanoparticles in an optically opaque medium due to scattering or absorption of light. Here, the electrochemical technique of ‘nano-impacts’ was used to detect nanoparticles in solution in the presence of high levels of alumina particulates causing a milky white suspension. Using the ‘nano-impacts’ method, silver nanoparticles were successfully detected and sized in the model opaque medium. The results obtained compared well with those using transmission electron microscopy (TEM), an ex situ method for nanoparticle size determination. The ability to use the ‘nano-impacts’ method in media unmeasurable to competitor techniques confers a significant advantage on the electrochemical approach.

As nanoparticles are defined by the International Union of Pure and Applied Chemistry (IUPAC) to be any materialw ith as ingle dimension below 100 nm, it is difficult to measurem aterials of such scale. [1] Yet, the size of the nanoparticle is ac rucial attribute because it can influence its properties. [2] For example,t he catalytic properties of an anoparticle can change with varying size. Small silver nanoparticles catalyse the reduction of oxygen to hydrogen peroxide insteado fw ater. [2b, c] With decreasing size, silver nanoparticles are also known to absorb at as maller wavelength and are electrochemically oxidised at al ower potential. [2d, e, 3] Although microscopy methods such as scanninge lectron microscopy (SEM) and transmission electron microscopy (TEM) are capable of resolving nanoparticles down to 3nma nd 0.1 nm, respectively;b oth of these are ex situ methods. [4] However,i ti si mportant that the nanoparticle size is measured in the solution phase as the removal of solvent could result in aggregation or agglomeration. [2e] Therefore, many techniques such as nanoparticle tracking analysis (NTA) and dynamic light scattering (DLS), which analyse nanoparticle size in the solution phase, have been developed.
NTAu ses al aser to illuminate the particles and am icroscope to detect the movement of the individual particles, whilst DLS measures the particles ize through the light scattered by the nanoparticles. [5] However,a no paque sample can contain large particulates, which can strongly affect both DLS and NTA measurements. In DLS, the opaque sample would scatter or absorb the majority of the light, thus, the light scattered by the nanoparticle would be overwhelmed. For NTA, as the laser shines on the sample, the particulates could be illuminated or they could absorb most of the light from the laser.T hus, the nanoparticles would remaini nt he shadow,m aking detection difficult or impossible.
Given that both NTAa nd DLS strongly depend on light to record their signals and the presence of large amounto fi nert macroparticulates can reflect or absorb mosto ft he light, DLS and NTAm easurement is effectively impossible in an optically opaque suspension.T herefore, there is an eed for at echnique that is capable of measuring nanoparticle size in an opaque medium. Anodic particle coulometry (via 'nano-impacts') is an ovel technique developed within the last 20 years that works on the basis of recording single nanoparticle-electrode impact events through electrochemistry. [6] These events are recorded though the electrochemical signal generated by the redox reaction occurring on the nanoparticle. In this case, the citrate-capped silver nanoparticle diffuses under Brownian motion and hits the carbon microelectrode held at as uitable oxidisingp otential. Thus, the silver nanoparticle is oxidised into silver(I)i ons, generating ac urrent 'spike' that is observed in the chronoamperogram recorded. Through the use of Faraday's first law,t he size of the nanoparticle can be estimated via Equation (1), [7] where R NP is the nanoparticle radius, Q is the total charge passed under as ingle 'spike', A r is the atomicm olecular mass of silver (107.9 gmol À1 ), F is the Faraday constant and 1 is the density of silver (10.5 10 6 gm À3 ). Therefore, if the opacityi sc aused by inert particles, this technique is capable of differentiating between the redox active nanoparticles and the inert particulates.
In this work, citrate-capped silver nanoparticles are detected in an optically opaque suspension that contains ahigh concentration of alumina particles. First, the oxidation potential of the citrate-capped silver nanoparticles is determined through anodic stripping voltammetry.S econd, 'nano-impact' experiments are performed in as uspension of silver nanoparticles, alumina particulates, and the electrolyte of sodium nitrate. The size distribution of silver nanoparticles is matched against in-dependentT EM measurements to evaluatet he size measured by 'nano-impacts' in the opaque medium. [8] Silver-nanoparticle-modified glassy carbon electrodes prepared as described in the Experimental Section were used to [ determine the oxidation potentialo fc itrate-capped silver nanoparticles in the presence of alumina powder (0.05 mm). An optically opaque electrolyte solution waso btained by suspending 0.25 % w/v alumina powder in 20 mm sodium nitrate solution.T he suspension has the coloura nd appearance of pure milk. Then, using voltammetric methods, the nanoparticle-modified electrode was scanned oxidatively from À0.6 V versus the mercury/mercurous sulfate reference electrode (MSE) in the opaque electrolyte. In Figure 1, it is seen that the silver oxidation signal occurs around + 0.05 Vv ersus MSE. [2e] The experiment was repeated three times to ensure reproducibility.I tw as inferred that the oxidation of metallic silver to silver(I) ions (Ag!Ag + + e À )i sn ot influenced by the presence of alumina powder.F romt he black dashed line in Figure 1, it is also concluded that alumina powder is inert under these conditions.
After determining the potential at which silver nanoparticles are oxidised in the opaque electrolyte, 'nano-impact' experiments were performed in the opaque solution.The opaque solution used for' nano-impact' experiment is depicted in Figure 2( sample on the right). The yellow silver nanoparticles present in the suspension causes it to appear as ay ellow milky suspension. 'Nano-impact' experiments were performed by using chronoamperometry.C urrent-time transients of af ixed duration (50 s) were recorded at an overpotential of + 0.6 V versus MSE. The results are summarised in Figure 3. In the absence of silver nanoparticles, no 'spikes' were observed;i n presence of silver nanoparticles, current 'spikes'were observed. This indicates that alumina powder did not give any 'spikes', no 'rogue' nanoparticles were present, and the 'spikes'o bserved are solely attributed to the silver nanoparticles.
In total, 498 spikes were recorded from 28 scans. The size distribution is depicted in Figure 4. The average radius of the nanoparticles was calculated to be 13.8 AE 2.2 nm. This is in excellent agreement with the TEM sizing of 14.6 AE 2.1 nm of the same batch of nanoparticles. [8] It can be concluded that the nanoparticles sizes are consistent andt hat 'nano-impact' experiments can be performed in opaque medium.
For this 'nano-impact' experiment, the frequency of 'spikes' is much lower compared with ap revious paper. [8] This is because the presence of particulates (alumina) leads to alarge increase in the amounto fp articles in solution,h ence making it harder for the nanoparticles to diffuse through the suspension. Therefore, the number of nanoparticle-electrode impact events decreases, reducing the number of 'spikes'observed.
In summary,' nano-impact' experimentsw erep erformed for silver nanoparticles in an optically opaque medium (a suspension of sodium nitratea nd alumina powder). Comparing the sizes of silver nanoparticles obtained from TEM and 'nanoimpact'e xperiments, the radiio btained were consistentw ith one another.The 'nano-impact' technique allows measurement
All electrochemical experiments were performed on at hree-electrode system in aF araday cage. The electrochemical experiments were controlled by a mAutolab II potentiostat from Metrohm-Autolab BV (Utrecht, The Netherlands) using NOVA1 .10 software. A glassy carbon electrode of 3.0 mm diameter from CH instruments (Austin, USA) was used as the working electrode for anodic stripping voltammetry.T he glassy carbon electrode was polished to am irror finish by using diamond sprays from Kemet International Ltd (Maidstone, UK) in the size sequence:3 .0 mm, 1.0 mma nd 0.1 mm. For anodic particle coulometry,acarbon microdisc electrode of radius 4.8 mmf rom BASi (West Lafayette, USA) was used as the working electrode. Prior to use, the microelectrode was polished by using alumina powder from Buehler (Coventry,U K) in the size sequence:1 .0 mm, 0.3 mma nd 0.05 mm. As tandard mercury/ mercurous sulfate reference electrode (MSE) [Hg/Hg 2 SO 4 ,K 2 SO 4 (saturated); + 0.62 Vv ss tandard hydrogen electrode] was obtained from BASi (WestLafayette, USA). [9] The counter electrode was aplatinum mesh (99.99 %) from Goodfellow Cambridge Ltd (Huntingdon, UK). All electrochemical measurements were thermostated at 25 AE 1 8C.
In order to perform anodic stripping experiments with as ilvernanoparticle-modified electrode, 3 mLofthe silver nanoparticle suspension supplied was dropcast on the glassy carbon electrode. For an alumina-powder-modified electrode, 3 mLo f5%w / vsuspension of alumina powder (0.05 mm) was dropcast on the glassy carbon electrode. The modified electrodes were dried under flowing nitrogen. After drying, the nanoparticle-modified electrode was immediately used to perform ac yclic voltammogram, sweeping from À0.6 Vt o+0.5 Vv ersus MSE at as can rate of 50 mV s À1 .
Prior to every 'nano-impact' experiment, the electrochemical cell was soaked in aqua regia (HCl/HNO 3 ,3 :1) for at least 30 min and sonicated in ultrapure water for 15 min to avoid any contamination by rogue nanoparticles. The silver nanoparticle suspension was diluted with 20 mm aq NaNO 3 containing 0.25 %( w / v )a lumina powder to give a1 00 pm solution of silver nanoparticles used for experiments. Chronoamperometric scans of 50 sd uration with as ampling time of 0.0005 sw ere recorded at ap otential of + 0.6 V versus MSE. The charge under each 'spike' was resolved by using SignalCounter to determine the size of the nanoparticle detected. SignalCounter was developed by Dr.D ario Omanović (Division for Marine &E nvironmental Research, Rud¯er Bošković Institutue, Zagreb, Croatia) for in-house use as part of acollaboration. [10]