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Ordered Monolayer Gold Nano-urchin Structures and Their Size Induced Control for High Gas Sensing Performance.

Sabri YM, Kandjani AE, Ippolito SJ, Bhargava SK - Sci Rep (2016)

Bottom Line: It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs.The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 μg/m3 at 75 °C.When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively.

View Article: PubMed Central - PubMed

Affiliation: Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne, VIC 3001 (Australia).

ABSTRACT
The synthesis of ordered monolayers of gold nano-urchin (Au-NU) nanostructures with controlled size, directly on thin films using a simple electrochemical method is reported in this study. In order to demonstrate one of the vast potential applications, the developed Au-NUs were formed on the electrodes of transducers (QCM) to selectively detect low concentrations of elemental mercury (Hg(0)) vapor. It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs. The Au-NU-12 min QCM (Au-NUs with nanospikes grown on it for a period of 12 min) had the best performance in terms of transducer based Hg(0) vapor detection. The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 μg/m3 at 75 °C. When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively. The results demonstrate the excellent activity of the developed materials which can be applied to a range of applications due to their long range order, tunable size and ability to form directly on thin-films.

No MeSH data available.


Modified (Au-NUs) and Au-control based QCMs’ (a) dynamic response, (b) response magnitudes (The solid fitted lines represent the best fits for the LRC equation) and (c) selectivity performance toward Hg0 vapor at 75 °C. The dynamic response was obtained when the QCMs were exposed toward Hg0 vapor concentrations of 0.21, 0.31, 0.45, 0.64, 0.93, 1.27, 1.74, 2.38, and 3.26 ± 0.05 mg/m3. The target concentration of 3.26 mg/m3 is represented by the solid magenta lines in each of the panels.
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f4: Modified (Au-NUs) and Au-control based QCMs’ (a) dynamic response, (b) response magnitudes (The solid fitted lines represent the best fits for the LRC equation) and (c) selectivity performance toward Hg0 vapor at 75 °C. The dynamic response was obtained when the QCMs were exposed toward Hg0 vapor concentrations of 0.21, 0.31, 0.45, 0.64, 0.93, 1.27, 1.74, 2.38, and 3.26 ± 0.05 mg/m3. The target concentration of 3.26 mg/m3 is represented by the solid magenta lines in each of the panels.

Mentions: The fact that the developed Au-NUs are easily synthesized as highly ordered nano-arrays directly on a thin-film and can cover large active surface areas; they could be excellent candidates for sensing applications. Among the numerous possible applications, the developed Au-NUs were used to address a globally urgent and important issue (i.e. detection of toxic mercury vapor) in order to demonstrate their chemical sensing performance. Figure 4 shows the Au-NU based sensors’ response toward Hg0 vapor at 75 °C as well as its selectivity performance when tested toward Hg0 vapor with/without the presence of common interferent gases (i.e. H2O, NH3, Ethyl-M, MEK, Ac-ald and DMDS, as defined in the supporting information) found in industrial effluents. The sensitivity of the Au-NU based sensors toward Hg0 vapor is found to increase with increasing nanospike size, up to an electrodeposition time of 10 mins (Fig. 3a,b). It is observed that any electrodeposition beyond 10 minutes did not significantly increase the sensor sensitivity. A comparison between Au-control and Au-NU-12 min reveals that the modified sensor has more than 5 times the sensitivity of the Au-control QCM. This sensitivity increase is more apparent (15.75 times the sensitivity) when the sensors are operated at 30 °C (see Supplementary information, Fig. S4) even though Au-NU-12 min has only ~8.5 times the surface area of the Au-control based Hg0 vapor sensor. The presence of crystal lattice as well as surface topology defects are well known to be sorption sites for Hg0 vapor205758. The relatively higher response magnitude of the Au-NUs toward Hg0 vapor is postulated to be due to the formation of such defect sites during the electrodeposition process when forming the nanospikes42. Therefore, although the Au-NUs was expected to have an even higher affinity toward Hg0 vapor due to increased number of defect sites, as noted in the Supplementary information, the Au-NU-15 min based QCM was dampened due to extensive growth of the nanospikes on the Au-MNM and so could not be tested. Regarding the sensing and adsorption-desorption of Hg0 on the gold surface, when fresh QCMs are exposed to Hg0 vapor, the first Hg0 pulse needs to be used as a pretreatment procedure (This is shown in Fig. S5). This process forms the initial Au-Hg amalgam layer on the surface of the Au film on QCM device. A portion of the initial frequency drop observed in the 1 hour Hg0 exposure step for both Au-control and Au-NU-12 mins is related to the formation of saturated amalgam layer on the surface of the Au structures. This portion is not recovered during the 1 hour N2 flushing and generation period since the operating temperature is maintained constant to enable chemical sensing experiments. The formation of this amalgam layer helps the surface to be saturated therefore allowing mercury atoms to only loosely adsorb to the surface during the Hg0 exposure event. As the regeneration process is introduced, these surface adsorbed Hg atoms do not have enough time to diffuse into the Au bulk and so most of the adsorbed Hg atoms desorb from the surface, thereby regenerating the surface. However, some diffusion of Hg into the Au surface is not avoidable due to Hg concentration gradient between the surface and bulk of the Au QCM electrode and so 100% regeneration can be a challenge. At higher temperatures, the Hg0 vapour pressure is higher and thus the desorption process occurs more readily. This is one of the main reasons for achieving better recovery at higher operating temperatures. Another important feature which is obtained by the formation of amalgam layer is the high reproducibility that can be achieved as shown in the Supporting information, Fig. S5. The Sauerbrey equation, which is a linear relationship between the frequency change of the QCM with that of the mass deposition, was used to determine the mass of Hg0 that was adsorbed and desorbed from the QCM surface during each pulse59 and presented in the right axis of Fig. S5. It can be observed that the sensor exhibited similar response magnitudes for each repeated concentration of Hg0 vapor tested and thus it operates more as an on-line sensor. In order to confirm that the developed Au-NUs based sensor response is solely due to their selective interaction with Hg0 vapor, they were tested toward six different interferent gases (listed in the Supplementary information, Table S1) with and without the presence of Hg0 vapor concentration of 3.26 ± 0.05 mg/m3 (Fig. 4c). The type and concentration of the interferent gases were specifically chosen to include those known for their high affinity towards Au surfaces as well as those commonly found in industrial effluents42. In order to show the extent of each sensor’s accuracy, solid magenta lines representing Hg0 vapor concentration of 3.26 mg/m3 is drawn in each panel. It can be observed that the sensor performance of the Au-NUs increased with increasing electro-deposition time and subsequent nanospikes size. The best accuracy was observed from the Au-NU-12 min based Hg0 vapor sensor (see the Supplementary information, Table S2) which showed 89% and 98% accuracy at ±10% (±0.33 mg/m3) and ±15% (±0.49 mg/m3) tolerance values, respectively. This excellent accuracy was further complemented with ~96% precision. The response time (based on 90% of full response magnitude) was estimated to be similar for all developed Au-NUs based sensors at approximately 48 minutes. Remarkably, the calculated LoD of the developed Au-NUs were found to be several fold lower than the Au-control based Hg0 vapor sensor with the largest tested Au-NU having the lowest LoD. That is, the Au-control and Au-NU-12 min based QCMs were found to have LoD of 149 and 32 μg/m3 at 75 °C, respectively. Based on the sensing data collected at 30 °C, the Au-NU-12 min showed and LoD of 16 μg/m3 which is the lowest LoD reported on QCM based Hg0 sensors reported as listed in Table 1. The relatively higher response magnitude and low LoD obtained with the developed Au-NUs as compared to other structures reported in literature is thought to have arisen from the presence of relatively large number defect sites (having high affinity toward Hg0 atoms) homogeneously formed throughout the whole sensitive layer due to growth of seamless spikes on ordered closely packed monolayer of Au-MNM. Furthermore, it was found that the Au-NUs based sensors had more than 90% recovery following each sensing event, without any external energy input thereby demonstrating their reusability over long periods of time. The results indicate that the developed Au-NUs based QCM sensors have high sensitivity and repeatability as well as being highly accurate toward determining low concentrations of Hg0 vapor in the presence of various interferent gas species. In addition to good sensitivity and selectivity, it is noteworthy that previous studies6061 have shown Au-QCM based Hg sensors can have lifetimes exceeding 1 year period before their signal-to-noise ratio is reduced to levels where the sensor is deemed unusable. This has been shown through long-term experimental analyses and theoretical calculations based on the stable amalgam that can be formed at the different operating temperatures. These results suggest that Au-NUs can be used as a potential future generation mercury sensor technology for numerous industrial applications.


Ordered Monolayer Gold Nano-urchin Structures and Their Size Induced Control for High Gas Sensing Performance.

Sabri YM, Kandjani AE, Ippolito SJ, Bhargava SK - Sci Rep (2016)

Modified (Au-NUs) and Au-control based QCMs’ (a) dynamic response, (b) response magnitudes (The solid fitted lines represent the best fits for the LRC equation) and (c) selectivity performance toward Hg0 vapor at 75 °C. The dynamic response was obtained when the QCMs were exposed toward Hg0 vapor concentrations of 0.21, 0.31, 0.45, 0.64, 0.93, 1.27, 1.74, 2.38, and 3.26 ± 0.05 mg/m3. The target concentration of 3.26 mg/m3 is represented by the solid magenta lines in each of the panels.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4835752&req=5

f4: Modified (Au-NUs) and Au-control based QCMs’ (a) dynamic response, (b) response magnitudes (The solid fitted lines represent the best fits for the LRC equation) and (c) selectivity performance toward Hg0 vapor at 75 °C. The dynamic response was obtained when the QCMs were exposed toward Hg0 vapor concentrations of 0.21, 0.31, 0.45, 0.64, 0.93, 1.27, 1.74, 2.38, and 3.26 ± 0.05 mg/m3. The target concentration of 3.26 mg/m3 is represented by the solid magenta lines in each of the panels.
Mentions: The fact that the developed Au-NUs are easily synthesized as highly ordered nano-arrays directly on a thin-film and can cover large active surface areas; they could be excellent candidates for sensing applications. Among the numerous possible applications, the developed Au-NUs were used to address a globally urgent and important issue (i.e. detection of toxic mercury vapor) in order to demonstrate their chemical sensing performance. Figure 4 shows the Au-NU based sensors’ response toward Hg0 vapor at 75 °C as well as its selectivity performance when tested toward Hg0 vapor with/without the presence of common interferent gases (i.e. H2O, NH3, Ethyl-M, MEK, Ac-ald and DMDS, as defined in the supporting information) found in industrial effluents. The sensitivity of the Au-NU based sensors toward Hg0 vapor is found to increase with increasing nanospike size, up to an electrodeposition time of 10 mins (Fig. 3a,b). It is observed that any electrodeposition beyond 10 minutes did not significantly increase the sensor sensitivity. A comparison between Au-control and Au-NU-12 min reveals that the modified sensor has more than 5 times the sensitivity of the Au-control QCM. This sensitivity increase is more apparent (15.75 times the sensitivity) when the sensors are operated at 30 °C (see Supplementary information, Fig. S4) even though Au-NU-12 min has only ~8.5 times the surface area of the Au-control based Hg0 vapor sensor. The presence of crystal lattice as well as surface topology defects are well known to be sorption sites for Hg0 vapor205758. The relatively higher response magnitude of the Au-NUs toward Hg0 vapor is postulated to be due to the formation of such defect sites during the electrodeposition process when forming the nanospikes42. Therefore, although the Au-NUs was expected to have an even higher affinity toward Hg0 vapor due to increased number of defect sites, as noted in the Supplementary information, the Au-NU-15 min based QCM was dampened due to extensive growth of the nanospikes on the Au-MNM and so could not be tested. Regarding the sensing and adsorption-desorption of Hg0 on the gold surface, when fresh QCMs are exposed to Hg0 vapor, the first Hg0 pulse needs to be used as a pretreatment procedure (This is shown in Fig. S5). This process forms the initial Au-Hg amalgam layer on the surface of the Au film on QCM device. A portion of the initial frequency drop observed in the 1 hour Hg0 exposure step for both Au-control and Au-NU-12 mins is related to the formation of saturated amalgam layer on the surface of the Au structures. This portion is not recovered during the 1 hour N2 flushing and generation period since the operating temperature is maintained constant to enable chemical sensing experiments. The formation of this amalgam layer helps the surface to be saturated therefore allowing mercury atoms to only loosely adsorb to the surface during the Hg0 exposure event. As the regeneration process is introduced, these surface adsorbed Hg atoms do not have enough time to diffuse into the Au bulk and so most of the adsorbed Hg atoms desorb from the surface, thereby regenerating the surface. However, some diffusion of Hg into the Au surface is not avoidable due to Hg concentration gradient between the surface and bulk of the Au QCM electrode and so 100% regeneration can be a challenge. At higher temperatures, the Hg0 vapour pressure is higher and thus the desorption process occurs more readily. This is one of the main reasons for achieving better recovery at higher operating temperatures. Another important feature which is obtained by the formation of amalgam layer is the high reproducibility that can be achieved as shown in the Supporting information, Fig. S5. The Sauerbrey equation, which is a linear relationship between the frequency change of the QCM with that of the mass deposition, was used to determine the mass of Hg0 that was adsorbed and desorbed from the QCM surface during each pulse59 and presented in the right axis of Fig. S5. It can be observed that the sensor exhibited similar response magnitudes for each repeated concentration of Hg0 vapor tested and thus it operates more as an on-line sensor. In order to confirm that the developed Au-NUs based sensor response is solely due to their selective interaction with Hg0 vapor, they were tested toward six different interferent gases (listed in the Supplementary information, Table S1) with and without the presence of Hg0 vapor concentration of 3.26 ± 0.05 mg/m3 (Fig. 4c). The type and concentration of the interferent gases were specifically chosen to include those known for their high affinity towards Au surfaces as well as those commonly found in industrial effluents42. In order to show the extent of each sensor’s accuracy, solid magenta lines representing Hg0 vapor concentration of 3.26 mg/m3 is drawn in each panel. It can be observed that the sensor performance of the Au-NUs increased with increasing electro-deposition time and subsequent nanospikes size. The best accuracy was observed from the Au-NU-12 min based Hg0 vapor sensor (see the Supplementary information, Table S2) which showed 89% and 98% accuracy at ±10% (±0.33 mg/m3) and ±15% (±0.49 mg/m3) tolerance values, respectively. This excellent accuracy was further complemented with ~96% precision. The response time (based on 90% of full response magnitude) was estimated to be similar for all developed Au-NUs based sensors at approximately 48 minutes. Remarkably, the calculated LoD of the developed Au-NUs were found to be several fold lower than the Au-control based Hg0 vapor sensor with the largest tested Au-NU having the lowest LoD. That is, the Au-control and Au-NU-12 min based QCMs were found to have LoD of 149 and 32 μg/m3 at 75 °C, respectively. Based on the sensing data collected at 30 °C, the Au-NU-12 min showed and LoD of 16 μg/m3 which is the lowest LoD reported on QCM based Hg0 sensors reported as listed in Table 1. The relatively higher response magnitude and low LoD obtained with the developed Au-NUs as compared to other structures reported in literature is thought to have arisen from the presence of relatively large number defect sites (having high affinity toward Hg0 atoms) homogeneously formed throughout the whole sensitive layer due to growth of seamless spikes on ordered closely packed monolayer of Au-MNM. Furthermore, it was found that the Au-NUs based sensors had more than 90% recovery following each sensing event, without any external energy input thereby demonstrating their reusability over long periods of time. The results indicate that the developed Au-NUs based QCM sensors have high sensitivity and repeatability as well as being highly accurate toward determining low concentrations of Hg0 vapor in the presence of various interferent gas species. In addition to good sensitivity and selectivity, it is noteworthy that previous studies6061 have shown Au-QCM based Hg sensors can have lifetimes exceeding 1 year period before their signal-to-noise ratio is reduced to levels where the sensor is deemed unusable. This has been shown through long-term experimental analyses and theoretical calculations based on the stable amalgam that can be formed at the different operating temperatures. These results suggest that Au-NUs can be used as a potential future generation mercury sensor technology for numerous industrial applications.

Bottom Line: It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs.The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 μg/m3 at 75 °C.When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively.

View Article: PubMed Central - PubMed

Affiliation: Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, GPO Box 2476V, Melbourne, VIC 3001 (Australia).

ABSTRACT
The synthesis of ordered monolayers of gold nano-urchin (Au-NU) nanostructures with controlled size, directly on thin films using a simple electrochemical method is reported in this study. In order to demonstrate one of the vast potential applications, the developed Au-NUs were formed on the electrodes of transducers (QCM) to selectively detect low concentrations of elemental mercury (Hg(0)) vapor. It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs. The Au-NU-12 min QCM (Au-NUs with nanospikes grown on it for a period of 12 min) had the best performance in terms of transducer based Hg(0) vapor detection. The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 μg/m3 at 75 °C. When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively. The results demonstrate the excellent activity of the developed materials which can be applied to a range of applications due to their long range order, tunable size and ability to form directly on thin-films.

No MeSH data available.