Heterogemini surfactant assisted synthesis of monodisperse icosahedral gold nanocrystals and their applications in electrochemical biosensing

Abstract

Icosahedral nanocatalysts (NCs) have shown very interesting physical and chemical properties owing to their multiply twinned nanostructures. Herein, we introduce a novel heterogemini surfactant (C₁₀OhpNC₈) assisted seed mediated growth approach for the synthesis of monodisperse icosahedral gold (Au) NCs in aqueous solution at room temperature. Very small shape impurities were observed in the resultant icosahedral Au NCs. Significantly improved monodispersity (relative standard deviation (RSD) of <10%) has been achieved by using a binary mixture of C₁₀OhpNC₈ and PVP as structure directing agents. Interestingly, the size of icosahedral Au NCs can be tuned ranging from 40 nm to 190 nm, which guides the surface plasmon resonance (SPR) peak to be tuned throughout the whole visible region and even to the near infrared (NIR) region. Furthermore, the developed icosahedral Au NCs specific probe has been designed to be applied as an easy electrochemical biosensor and successfully used to detect the bacteria Escherichia coli O157:H7 (E. coli O157:H7) with a detection limit of ∼10 colony forming units (CFU) mL⁻¹. Notably, a much higher sensitivity of these icosahedral Au NCs probes has been achieved compared to the traditional colloidal gold immunochromatography (detection limit ∼10³ CFU mL⁻¹).

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1. Introduction

The synthesis of gold nanocrystals (NCs) with precise size and shape has continually attracted intensive research interest in diverse fields ranging from optical sensing,^1–3 optoelectronics, and catalysis^4–6 to cancer theranostics,^7,8 owing to their intriguing size/shape dependent optical, electronic and catalytic properties.^9,10 To date, tremendous efforts have been focused to offer efficient methods for obtaining size and shape controllable gold nanostructures including nanowires,¹¹ nanoplates,^12–14 nanorods,^15–19 nanospheres^20,21 and nano-polyhedrons.^22,23 In this direction, platonic (i.e. tetrahedron, octahedron, hexahedron, icosahedrons and dodecahedron) nanocrystals, constructed by congruent regular polygonal faces with the same number of faces meeting at each vertex, are by themselves a scientific wonder due to their very high activities in several applications. Among the platonic nanocrystals, icosahedron comprising highest numbers of twin boundaries (30) and faces (20) are considered to be a superb candidate to show most promising catalytic and electrocatalytic activities in addition to their unique optical properties. Icosahedral NCs comprises high-index facets^22,24 with sharp corners and edges as well as multiple twins,^25,26 which strongly influence the electro-catalytic activities due to abundance of active surface sites in their structure. These unique structural properties also enable icosahedral Au NCs to be applied in several other applications including catalytic, surface enhancement spectroscopy and chemical or biological sensing.

Over the last decade, several researches were focused on synthesizing highly dispersed icosahedral Au NCs and most of them were synthesized via thermal polyol process.^26–28 For example, Yang and co-workers have reported icosahedral Au NCs of 230 nm diameter using ethylene glycol at 280 °C and N₂ atmosphere in the presence of poly(vinylpyrrolidone) (PVP).²⁹ A few water-based systems also have been reported during previous decades. Han group produced 10–90 nm tunable gold icosahedron in 0.1 M hexadecyltrimethylammonium bromide (CTAB) solution by a seed-mediated growth approach.³⁰ Richards and co-workers prepared 30–250 nm size controlled utilizing glucose as reductant and sodium dodecyl sulfate as directing agent.³¹ Yang and co-workers developed alternating voltage induced electrochemical synthesis to get access to 14 nm Au icosahedron.³² Although it has been possible to synthesize the icosahedrons Au NCs via above methods, however, there exist several disadvantages including non-uniformity in size/shape or utilization of high temperature, organic solvent and high-concentration of surfactants to originate cytotoxicity. This has motivated us to study and endow with a unified method to obtain size tunable monodisperse icosahedral Au NCs through a simple and greener approach.

It was indeed a great challenge to find a suitable method to obtain size tunable monodisperse icosahedral Au NCs, which requires delicate control over the growth of the NCs. In general, surfactants play key role to control the growth process and minimize total surface energy of the particles to gain the stability. To minimize the total surface energy, every faces of icosahedral NCs are usually enclosed with {111} facets. Therefore it is highly important to use proper surfactant to block the {111} facets of a seed and restrict the growth along {111} direction and consequently all the faces of the nanocrystals will be enclosed by the {111} facets to form icosahedral shaped particles. Herein, we have used a novel heterogemini surfactant N,N-dimethyl-N-[3-(alkyloxy)-2-hydroxypropyl]-alkylammonium bromide, referred to as C₁₀OhpNC₈ to control the growth of Au NCs to obtain monodisperse icosahedral (yield ∼ 70%) shaped particles via a seed-mediated growth approach in aqueous solution at room temperature. Notably, five times lower concentrations of C₁₀OhpNC₈ (at low as 0.02 M) was maintained in the growth solution compared to the standard surfactant CTAB (0.1 M) to make it “eco-friendly” method to be applied in the future development of green chemistry. As described earlier,²⁶ PVP play an important role in the growth of polyhedral nanocrystals. Inspired by knowledge on the complex phase behavior of binary surfactants mixtures from the surfactant science community and various efforts to elucidate seed-mediated growth mechanism,^33–35 we employed the possibility of PVP in combination with C_mOhpNC_n for improved synthesis of high-quality gold icosahedrons and successfully achieved the improved yield of >90%. Moreover, size of the icosahedral Au NCs has been tuned from 40 to 200 nm by controlling the amounts of seed and growth solution. This size tenability also helps to control the position of surface plasmon resonance throughout the whole visible region and even towards the near infrared (NIR) region. Furthermore the electrochemical activities of the developed icosahedral Au NCs were also investigated using Escherichia coli O157:H7 (E. coli O157:H7) as a model target analyte. To our best knowledge, no icosahedral Au NCs based electrochemical biosensor has been previously reported to detect E. coli O157:H7. Our method is simple and rapid, however, does not need any expensive equipment. Nevertheless, the sensitivity of this method is much higher compared to the conventional colloidal gold immunochromatography.^36,37

2. Experimental section

2.1. Materials and instruments

Hexadecyltrimethylammonium bromide (CTAB, >99.0%) and hydrogen tetrachloroaurate trihydrate (HAuCl₄·3H₂O, 99.99%) were purchased from Sigma-Aldrich. Silver nitrate (AgNO₃, >99.0%) and poly(vinylpyrrolidone) (PVP, ∼5500 kg mol⁻¹) were purchased from Aladdin. All other reagents and solvents were analytical reagent grade and were used as received from Sinopharm Chemical Reagent. Ultraviolet-visible (UV-vis) absorption spectra were recorded with TU-1810 spectrophotometer from Beijing Purkinje General Instrument Co. Ltd. in transmission mode. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100F instrument and operated at 200 kV. Scanning electronic microscopy (SEM) measurements were carried out by a JEOL JMS-6700F scanning microscope. Electrochemical measurements were carried out in a conventional three-electrode system using a CHI660E electrochemical workstation (Shanghai Xi'aiqu Instruments, China).

2.2. Synthesis of heterogemini surfactants

N,N-Dimethyl-N-[3-(alkyloxy)-2-hydroxypropyl]-alkylammonium bromide, referred to as C₁₀OhpNC₈ was synthesized as previously reported.³⁸ The product was recrystallized from ethyl acetate, and white solids were collected. Purified product was validated by ¹H NMR after sufficiently dried. The chemical structures are shown in Fig. S1.†

2.3. Preparation of icosahedral Au seeds (12 nm)

The Au seeds (12 nm) were prepared according to the publication.³⁹ Briefly, 36 mL of an aqueous solution of 2.5 × 10⁻⁴ M HAuCl₄ was prepared in a conical flask followed by the addition of 4 mL of 1 M D-glucose. The mixture was heated to 60 °C in an oil bath. Then, 80 μL of 1 M NaOH was added into the mixture, and the resulting solution was rapidly stirred for 10 s. The colour of the solution was immediately changed to ruby red, indicating the formation of Au nanoparticles. The average size of the seed Au nanoparticles was approximately 12.2 ± 3.5 nm. The Au-seed solution was cooled and used without modification for further experiments.

2.4. Growth of icosahedral Au nanocrystals

The growth solutions were prepared as follows. In 20 mL conical flasks, 9 mL aliquot of the growth solution containing a mixture of 2.5 × 10⁻⁴ M HAuCl₄ and 0.02 M C₁₀OhpNC₈ was first transferred. Then, 50 μL of 0.1 M freshly prepared ascorbic acid was added into each flask followed by gentle stirring for 2 min. Finally 0.5 mL of Au seed solutions (12 nm) was added into each flask, and the mixtures were kept at 30 °C in a water bath at least 6 h. The final solution centrifuged 5 times at 5000 rpm for 10 min.

Another series of experiments were performed to study the influence of binary surfactants mixtures PVP and C₁₀OhpNC₈ with varying contents on the purity of synthesized icosahedral Au NCs. Six 20 mL conical flasks were taken. A 9 mL aliquot of growth solution containing a mixture of 2.5 × 10⁻⁴ M HAuCl₄, 0.02 M C₁₀OhpNC₈, PVP with different weights (1, 50, 75, 100, 200 and 600 mg, respectively) was added to each of these flasks. Then, 50 μL of 0.1 M freshly prepared ascorbic acid was added into each flask followed by gentle stirring for 2 min. Finally 0.5 mL of Au-seed solution was added into each flask, and the mixtures were kept at 30 °C in a water bath for at least 6 h. Similarly, the influence of temperature on the purity of the synthesized icosahedral Au NCs was studied.

Au icosahedron with different sizes ranging from 40 to 200 nm were prepared using one-step seeding growth method by varying the volume ratios (2 : 1; 1 : 5; 1 : 15; 1 : 30; 1 : 35; 1 : 45) of growth and seed solution. An optimized growth solution was prepared by addition of 4.44 mL of 0.1 M ascorbic acid into 200 mL of 0.01 M CTAB and 2.5 × 10⁻⁴ M HAuCl₄. The concentration of seed solution was kept at 1 mL in each plastic tube while different amounts of growth solution (0.5 mL, 5 mL, 15 mL, 30 mL, 35 mL and 45 mL) were added into each tube. The solutions were kept at 30 °C in a water bath for at least 6 h. The final solution centrifuged 5 times at 5000 rpm for 10 min.

2.5. Electrochemical characterization

The cyclic-voltammetry (CV) measurements were performed as the reported method.^22,40 1 mL of 1 mM icosahedral Au NCs solution was washed five times by centrifugation, then pipetted 20 μL of the solution onto GCE and served as the working electrodes. A Pt wire and Ag/AgCl (in saturated KCl) were employed as the counter and the reference electrodes, respectively. 10 mL of 0.1 M H₂SO₄ aqueous solution, purged 30 min by high-purity N₂ gas, was used as the electrolyte solution. The voltage scan rate was 20 mV s⁻¹. The experimental parameters were as follows: initial potential: 1.6 V; final potential: −0.2 V; pulse amplitudes: 0.05 V; pulse width: 0.05 s; sample width: 0.0167 s.

Electrochemical activities of icosahedral Au NCs modified GCE were performed to establish a method for the detection of bacteria, E. coli O157:H7. Prior to GCE modification, 1 mL of 1 mM icosahedral Au NCs solution was washed five times by centrifugation. Seven numbers of GCE were taken. A 20 μL of the icosahedral Au NCs solution was pipetted onto the every GCE and dry at room temperature. Then, 6 μL of 1.1714 mM antibody and 5 μL of 5% Nafion were dropped onto the electrode surface and incubated for 30 min at RT. 100 mL of 0.01 M phosphate buffer saline (PBS, pH = 7.4) stock solutions was prepared. Seven 20 mL conical flasks were taken. Next, different concentrations of E. coli O157:H7 with ranging from 0 to 1 × 10⁷ CFU mL⁻¹ in 10 mL of 0.01 M PBS (pH = 7.4) buffer solution were added to these flasks and used as the electrolyte solution. The electrochemical experiment parameters were as follows: initial potential: 1.6 V; final potential: 0.6 V; pulse amplitudes: 0.05 V; pulse width: 0.05 s; sample width: 0.0167 s.

3. Results and discussion

3.1. Characterisation of Au NCs

First we prepared the surfactant C₁₀OhpNC₈, which can promote the growth of wormlike micelle to facilitate the formation of icosahedral Au NCs. Electron microscopic images of the icosahedral gold prepared by a seed-mediated method (9 mL of growth solution containing 0.02 M C₁₀OhpNC₈, 2.5 × 10⁻⁴ M HAuCl₄ and 5.55 × 10⁻⁴ M ascorbic acid mixed with 0.5 mL of 12 nm Au-seeds) have shown in Fig. 1. SEM image of the NCs reveals the formation of icosahedral shaped particles (average size ∼ 44 nm) with sharp corner and edges. TEM image of NCs also supports the formation of icosahedral NCs, however, a few spherical gold nanoparticles were also observed. The population (or yield) of icosahedral shaped particles was estimated to be ∼70% based on the analysis of TEM/SEM micrographs. Higher-magnification of SEM and TEM images, as shown in the insets of corresponding figures, clearly resemble the icosahedral shaped particle. It is well established in the literature³¹ that a typical icosahedrons covered with {111} facets at the faces and multiple twins at the boundaries.

Fig. 1 Representative TEM (A) and SEM (B) images of the icosahedral Au NCs (44 nm) using C₁₀OhpNC₈ as surfactant in growth solution.

To shed light on the mechanism icosahedral Au NCs growth in C₁₀OhpNC₈, the microstructures and surfactant's aggregation behaviour were studied. It is known to all that C₁₀OhpNC₈ has two different hydrophilic head group associated with one Br anion and two different hydrophobic tail, reducing the electrostatic repulsion between head groups, further promoting the transition from spherelike to rodlike or wormlike micellar.^33,41 The critical micelle concentration (CMC) is a great important factor affecting the final size and shape of Au NCs. According to previous studies,^25,42 larger CMC has lower free energy, inducing insufficient arrangement of surfactant micelle, while small CMC leads to stronger driving forces, resulting in excessive surface coverage and further preventing the growth of icosahedrons. The obtained heterogemini surfactant (C₁₀OhpNC₈) has a larger CMC of 1.42 mM than CTAB (0.98 mM), favoring to the synthesis of gold icosahedra. On the other hand, compared with conventional surfactants such CTAB or CTAC, the heterogemini surfactant with significant molecular structure, and the higher hydrophobicity would decrease (five times) the used concentration of surfactant. And the well-defined icosahedral gold can be synthesized at lower concentration.

It is also known that different facets of Au NCs have different adsorption and desorption properties towards different surfactants. According to the literature,⁴³ surfactant molecules tend to bind strongly to the {100} than the {111} faces. Thus, higher concentration surfactant conditions enable the Au seeds grow to both {100} and {111} faces (Fig. S2A†), leading to nonspherical shapes such as rods, and plates; while lower concentration of surfactants induces the faster deposition of Au⁰ onto the {111} faces, resulting in rod shapes. And lower concentration also induces the homogeneous nucleation, leading to the formation of smaller spheres (Fig. S2B†). The experimental results indicate neither too high nor low concentration of surfactant is favorable to the high yield growth of Au NCs.

Amine species can decrease the surface energy of gold remarkably attributed to the strong adsorption and thus regulate the growth of icosahedra {111} facets. Br atoms prefer to adsorb above gold surface site, and affect more energy changes than in the case of alkyl-ammonium cation. Thus, Br atoms play an important role in determining the end point of Au particles due to its preferential adsorption.

Further improvement in the yield of icosahedral Au NCs was achieved by a method inspired by previous studies of using binary surfactants to obtain different shaped Au NCs with improved uniformity. Herein, we exploit the possibility of using C₁₀OhpNC₈ and PVP as the binary system for improving the purity of icosahedral Au NCs. PVP molecules can attach to the surface of the Au NCs through their N–C=O groups and affect the growth rate of the different facets of the NCs. It has been described that selective interaction between PVP and different planes of Au NCs can guide the growth along {111} direction, which would facilitate the formation of icosahedral shaped NCs. For that purpose, 50 mg of PVP was added within the growth solution (9 mL) containing 0.02 M C₁₀OhpNC₈, 2.5 × 10⁻⁴ M HAuCl₄ and 5.55 × 10⁻⁴ M ascorbic acid, followed by the addition of 0.5 mL of 12 nm Au-seeds. The developed NCs were characterized by TEM and presented in Fig. 2, which confirmed the formation of icosahedral Au NCs with average size ∼57 nm. The yield of icosahedral shaped particles significantly improved and estimated to be >90%. A high magnification TEM image, as shown in Fig. 2B, demonstrates that the most of the Au NCs are nearly hexagonal-based pyramidal shape. Combination of C₁₀OhpNC₈ and hydrophilic PVP formed a homogeneous aqueous solution owing to the penetration of PVP into the palisade layer of CTAB micelles and the N–C=O group of PVP could access to the interlayer between C₁₀OhpNC₈ head group and gold NCs surface to favour the growth of icosahedral NCs. This is likely to be caused by the synergistic electrostatic and hydrophilic interaction.

Fig. 2 Representative TEM images with different magnifications (A and B) of the icosahedral Au NCs (57 nm) using 0.02 M C₁₀OhpNC₈ and 50 mg PVP as binary surfactants.

To shed more light on the synergistic effect of binary system, we synthesized the Au NCs using PVP as sole structure directing agent keeping all other conditions identical. The nanostructures obtained by the growth of 0.5 mL 12 nm seeds in the growth solution containing 50 mg PVP, 2.5 × 10⁻⁴ M HAuCl₄ and 5.55 × 10⁻⁴ M ascorbic acid are shown in Fig. S3.† The image appears to have non-uniform size and morphologies of Au NCs probably due to the deficiency of selective adsorption.

The UV-vis-NIR absorption spectrum of the samples obtained before and after adding PVP in growth solution for growing icosahedral Au NCs is shown in Fig. S4.† It is well established that surface plasmon resonance (SPR) absorption of the NCs precisely depends on their size, shape and uniformity. We found narrower absorption spectrum for the NCs obtained by using PVP/C₁₀OhpNC₈ binary mixture compared to only C₁₀OhpNC₈ as structure directing agent. Narrower width at the half maxima of the absorption spectrum, using binary systems, further confirms the narrower size distribution i.e. higher uniformity of the Au icosahedrons, which supports the results of TEM analysis (Fig. 1 and 2). Absorption spectra of the NCs exhibited the SPR peaks at 532 and 535 nm for samples A (44 nm) and B (57 nm), respectively. Absorption bands of sample B undergo slight red-shifting in the spectra owing to larger size of the NCs.

In order to understand the effect of PVP, a series of experiments by controlling the amounts of PVP (1 mg, 50 mg, 75 mg, 100 mg, 200 mg and 600 mg) were performed and the resultant nanostructures were characterized in TEM (Fig. 3). As shown in Fig. 3a, there is no obvious change when very low amount of PVP took part in the reaction. When the amounts of PVP was increased to 50–75 mg a higher yield (>90%) of icosahedral Au NCs was observed as revealed in Fig. 3b and c. However, further increase in the amounts of PVP gradually decreases the yield of the icosahedral shaped particles with the formation of little other polyhedral morphologies (Fig. 3d–f). This may be due to the fact that PVP only can accelerate the formation of polyhedrons without any significant effect on the uniformity of particles at room temperature. This fact probably forces the previous researchers to use high temperature (>100 °C) to produce uniform icosahedral Au NCs using PVP as capping agent. From the above results, we endow with an optimum condition by using a binary mixture of PVP (50–75 mg) and C₁₀OhpNC₈ (0.02 M) to obtain highest yield of the icosahedral shaped particles at room temperature, which is one of our prime concerns to make the process greener.

Fig. 3 Representative TEM images of the icosahedral Au NCs using different amounts PVP in 0.02 M C₁₀OhpNC₈. The amounts of PVP were varied as: (A) 1 mg, (B) 50 mg, (C) 75 mg, (D) 100 mg, (E) 200 mg and (F) 600 mg.

Thereafter, the as-synthesized uniform icosahedral Au NCs were used as seeds for further growth of the NCs in a one-step seeding approach to synthesize larger NCs keeping the shape unchanged. An optimized growth solution which favoured the isotropic growth condition to retain the icosahedral shape of the seeds in the final structure, was prepared by adding of 4.44 mL of 0.1 M ascorbic acid in a mixture of 0.01 M CTAB, 2.5 × 10⁻⁴ M HAuCl₄. Six sets of experiments were carried out by adding a fixed amount of seed solution (1 mL) in varying amounts (0.5 mL, 5 mL, 15 mL, 30 mL, 35 mL and 45 mL) of growth solution in six different vials. The developed nanostructures were characterized by TEM and UV-vis-NIR spectrometer. Fig. 4 shows TEM images of Au icosahedrons synthesized by varying the volume ratios of seed and growth solution (2 : 1; 1 : 5; 1 : 15; 1 : 30; 1 : 35; 1 : 45) and corresponding average size of the NCs were estimated to be 65 nm, 97 nm, 139 nm, 161 nm, 168 nm and 183 nm, respectively. In this way, the size of the icosahedral Au NCs can be well controlled in the range of 44–183 nm.

Fig. 4 Representative TEM images of the grown icosahedral Au NCs with different sizes: (A) 65 nm; (B) 97 nm; (C) 139 nm; (D) 161 nm; (E) 168 nm; (F) 183 nm.

Optical properties of the developed NCs were shown in Fig. 5, which exhibits the UV-visible absorption spectra of corresponding icosahedral NCs obtained in Fig. 4A–F. The color of the solution A–F (inset of Fig. 5) was gradually changed from purplish red into purple and finally becomes fade. For the NCs with average size up to 161 nm (solutions A–D), a single absorption peak has been appeared at 538 nm, 561 nm, 603 nm, 632 nm, respectively in their corresponding spectrum. However, sample E displays two absorption peaks emerged at 560 nm and 657 nm. The stronger absorption peak appeared at 657 nm can be ascribed to the dipole plasmon resonance of the gold icosahedrons, and relatively lesser intense peak at 560 nm contributed by the quadrupole plasmon resonance, which is consistent with previous reports. The spectra of the solution F also presents two distinct absorption bands at 562 and 735 nm, corresponding to the quadrupole and dipole resonances of icosahedral Au NCs, respectively. Therefore it has been observed that the increase in the size of icosahedral Au NCs causes dramatic red-shifting with relative broadening in their spectrum and eventually the single band evolves into two divisive bands for much larger NCs. In this way we can tune the absorption peaks of the NCs throughout the whole visible region, even upto NIR region by controlling the size of the NCs.

Fig. 5 UV-vis spectra of the grown icosahedral Au NCs with different sizes corresponding to Fig. 4.

3.2. Electrochemical investigation of icosahedral gold molecule

In addition to the beautiful optical properties, icosahedral Au NCs having multiple-twinned nanostructures with high surface-defect density could show enhanced electrochemical activities to be applied in chemical or biological sensing. The advanced electrochemical properties of icosahedral Au NCs (average size ∼ 57 nm) was evaluated through CV measurements as shown in Fig. 6. The CV trace of icosahedral gold (curve B) represents an oxidation peak around 1.29 V, whereas it appears at 1.37 V for spherelike gold nanoparticles. The negative shift is 0.08 V for the icosahedrons. The reduction peak potential of icosahedron occur at −0.85 V, while it is at −0.86 V for spherelike Au particles and corresponding currents are −0.523 mA and −0.395 mA, respectively. These results confirmed that icosahedron Au NCs with multiple twinned structure exhibit higher electrochemical activity than the sphere-like Au NCs.

Fig. 6 CV traces of the (a) gold nanosphere (b) icosahedral Au NCs in the 0.1 M H₂SO₄ aqueous solution.

To study the electrochemical advancement of icosahedron Au NCs, it was employed as a biosensor to detect E. coli O157:H7. The fabrication protocol is revealed in Fig. 7. Electrochemical detection of E. coli O157:H7 was investigated by varying the peak current intensity of signal molecules, presented in Fig. 8. Eight different concentrations of E. coli O157:H7: (a) 1 × 10⁷; (b) 1 × 10⁶; (c) 5 × 10⁴; (d) 2 × 10⁴; (e) 1 × 10⁴; (f) 1 × 10³; (g) 0 CFU mL⁻¹, in the 0.01 M PBS (pH = 7.4) buffer were used as the electrolyte solution. In Fig. 8A, it can be easily observed that the current intensity of CV curves increased with decreasing E. coli O157:H7 concentrations. The plot of CV peak current and the logarithm of concentrations of E. coli O157:H7 exhibit an almost linear relationship following I (mA) = 6.30 × 10⁻⁴ − 3.97 × 10^x−5 with a relevant coefficient of 0.965. The limit of detection (∼10 CFU mL⁻¹) was defined via the standard deviation derived from the repeated CV measurements of the bare 0.01 M PBS solution. This lower detection limit indicates that the icosahedral Au NCs based biosensor is more sensitive than traditional colloidal gold immunochromatography.⁴⁴

Fig. 7 Schematic for icosahedral Au NCs assembled as an electrochemical biosensor for detection of E. coli O157:H7.

Fig. 8 (A) CV curves of the electrochemical biosensor after reaction with different concentrations (in CFU mL⁻¹) of E. coli O157:H7 in the 0.01 M PBS solutions (pH = 7.4): (a) 1 × 10⁷; (b) 1 × 10⁶; (c) 5 × 10⁴; (d) 2 × 10⁴; (e) 1 × 10⁴; (f) 1 × 10³; (g) 0. (B) Linearity curve for the plot of CV peak current vs. the logarithm of concentration of E. coli O157:H7.

4. Conclusions

We have successfully utilized a novel heterogemini surfactant (C₁₀OhpNC₈) to synthesize monodisperse icosahedral Au NCs (yield ∼70%) by seeded growth method. The purity can be further enhanced (population of icosahedrons >90%) by using a binary mixture of C₁₀OhpNC₈ and PVP as structure directing agents. The size of icosahedral Au NCs can be tuned from 44 to 183 nm by varying the volume ratios of seed to growth solution. The developed nanostructures have shown very interesting optical properties by showing tunable SPR peak throughout the whole visible range and even to NIR region by controlling the size of the NCs. We also demonstrated the advantage of this nanostructure for the development of an electrochemical biosensor to detect the bacteria E. coli O157:H7, which is more sensitive than traditional colloidal gold immunochromatography.

Acknowledgements

We thank the Chinese Academy of Science for Hundred Talents Program, Chinese Central Government for Thousand Young Talents Program, the Natural Science Foundation of China (21376054, 21404110, 51473179, 51303195, 21304105), Excellent Youth Foundation of Zhejiang Province of China (LR14B040001), Ningbo Science and Technology Bureau (Grant 2014B82010 and 2015C110031), Youth Innovation Promotion Association of Chinese Academy of Sciences (2016268), the Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (2015).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03348b

‡ These authors contribute equally to this work.

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