Introduction

Gold nanoparticle (AuNP) assemblies have attracted considerable attention because of their outstanding properties, which are applicable to devices in fields such as electronics1, magnetics2 and plasmonics3. Various methodologies have been developed to produce assembled structures, such as polymer-mediated structures4,5, structures with different particle size6 and lithographically-defined template structures7,8. One of the most sophisticated methods to construct a AuNP assembly structure is to use DNA as a building block, which enables arrangement of nanoparticles in three dimensions9. However, the physical properties can also be controlled by modifying the stabilization mechanism, as well as the electronic and/or magnetic properties of the organic molecules protecting nanoparticles. The physical properties of gold nanoparticle assemblies, as well as the stability and size distribution of the particles, are determined by stabilization mechanisms through interaction with organic molecules. Interactions between gold nanoparticles and molecules are mainly classified into two types: covalent bonding and physisorbed interaction between the particle surface and molecules. Covalent bonding protection has been mainly used to synthesize gold nanoparticles because the chemical bonding between the gold atom and the molecule, such as Au–S, provides enough stability to allow the facile synthesis of thermally stable particles with a narrow size dispersion. On the other hand, physically protected nanoparticles have attracted practical interest from the view-point of weak interactions between molecules and nanoparticles. For example, catalytic reactions10 and cell recognition11 take advantage of surface accessibility of reactive species or cells. Fabrication of an electrical circuit through a solution-based process can also be achieved using weakly stabilized metal nanoparticles because the removability of the surface ligand permits a decrease in the sintering temperature, which is a crucial factor from the view-point of energy cost12 and field-effect transistor performance13. Physically stabilized nanoparticle assemblies are also favourable for achieving single-electron tunnelling14 because physisorbed molecules prevent strong electronic coupling between nanoparticles and/or nanoparticle–molecule coupling that cause an increase in conductance and a decrease in the tunnelling decay constant15.

Several approaches have been attempted to stabilize gold nanoparticles through physisorption using specific binding groups such as carboxylates, amines and pyridine. For example, Griffin and Fitzmaurice found that 4-(dimethylamino)pyridine was adsorbed normal to the surface of the gold nanoparticle through electrostatic interaction with the nitrogen atom of the pyridine ring16. An aqueous polymer, poly(N-vinyl-2-pyrrolidone), was also able to stabilize gold nanoparticles through multiple co-ordination of the nitrogen and oxygen atoms of the pyrrolidone units4. Unexpectedly, thienyl8 or acetylthio group17 substitution of porphyrin derivatives at meso positions stabilized gold nanoparticles through electrostatic or multiple Au-S interactions which is much weaker than thiol-gold covalent bond. However, we could not completely eliminate the possibility of covalent bond formation between gold surface with these molecules at the chalcogen-atom sites.18 Motivated by these results, we adopted π-conjugated macrocycles as stabilizers of gold nanoparticles. Stabilization of gold nanoparticles with a π-conjugated macrocycle reduces the particle–molecule or particle–particle distance and permits incorporation of molecules with high dielectric constants between gold particles while keeping electronic coupling between them weak. Therefore, we can expect, for example, to observe collective single-electron tunnelling8 or nanoscale inductor or capacitor behaviours3.

Here we present the novel gold nanoparticle assemblies which stabilized by neutral bis(phthalocyaninato)lanthanide(III) (LnPc2) complexes through v. d. Waals interaction (Figure 1). These complexes have two distinct features: 1) the complexes have no specific binding units able to connect onto the gold surface covalently. In addition, electrostatic interaction between neutral molecules and nanoparticles should be small. Instead, the conjugated π system of phthalocyanine ligands is expected to act as the binding site. 2) Electronic and magnetic properties of the complexes can be controlled easily. For example, the energy level of the frontier orbitals can be modified by introducing electron-withdrawing/donating groups to the phthalocyanine ligand or by extending the π system. Moreover, the magnetic properties of the complexes drastically change by changing the lanthanide ion from diamagnetic and paramagnetic to that of a single-ion magnet. Here, we employ two neutral complexes: bis(phthalocyaninato)lutetium(III) (LuPc2) and bis(phthalocyaninato)terbium(III) (TbPc2). The former is diamagnetic (Lu3+:1S0), whereas the latter is known to be a single-ion magnet, whose magnetic moment arises from Tb3+ (7F6). By employing LnPc2 complexes as a protecting ligand for gold nanoparticles, inter-nanoparticle distance was sufficiently close while the electronic nanoparticle-nanoparticle and nanoparticle-complex interactions remained weak. Such weak electronic interactions within the assembly prevented the disappearance of plasmonic or semiconducting properties19. This implies that the LnPc2–AuNP assemblies provide the opportunity to control the physical properties by molecular design rather than by the arrangement of nanoparticle arrays. These gold nanoparticle assemblies will open up new applications such as the realization of the biomimesis of an artificial neural network brain whose elements are integrated through weak interactions.

Figure 1
figure 1

Synthesis scheme of bis(phthalocyaninato)lanthanide(III) protected gold nanoparticles (LnPc2–AuNP, Ln = Lu, Tb).

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Results

Characterization of gold nanoparticle assemblies stabilized by bis(phthalocyaninato)lanthanide(III) complexes

Figure 2 shows TEM images of LuPc2–AuNP and TbPc2–AuNP. Both LuPc2–AuNP and TbPc2–AuNP were observed as black aggregates with a size of several hundred nm. Each aggregate was composed of monodispersed Au particles with diameters of about 4 nm connected with each other (Figure 2 inset). Isolated particles were not observed. Due to very poor solubility of LnPc2–AuNPs in ordinary solvents, we could not obtain, NMR, electronic (absorption, diffuse reflectance) and vibration spectra (IR, Raman).

Figure 2
figure 2

TEM images of (a) LuPc2–AuNP, (b) TbPc2–AuNP.

Enlarged images are in the upper right corner.

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In the aggregates, nanoparticles were arranged randomly, showing no periodic arrangements. Random particle arrangements have often been observed in aggregates connected by strong interactions between particles, such as nanoparticles covalently linked by alkane dithiol20. On the other hand, regularly arranged nanoparticles with closed packing structures have been achieved through weak van der Waals interactions working between particle to particle surfaces driven by, for example, the hydrocarbon chain of dodecanethiol21, in which the nanoparticles can rearrange to energetically minimum arrangements. The random arrangements of both LuPc2–AuNP and TbPc2–AuNP would be caused by π−π interaction of phthalocyanine ligands adsorbed on the surface of the gold nanoparticles.

We estimated the number and the molecular orientation of LnPc2 molecules adsorbed on the gold nanoparticles. By avoiding the aggregated area observed in the TEM images, we estimated the diameter of the gold nanoparticles as 3.7±1.2 nm and 4.2±1.4 nm for LuPc2-AuNP and TbPc2-AuNP, respectively (see Supplementary Fig. S1). Based on the assumption that each gold nanoparticle is composed of dense-packed Au atoms22 with 7 shells, we estimated that the number of atoms in one gold nanoparticle was 1.4×103 gold atoms, which can be obtained from the formula Σ10n2+2, with n being the atomic shell number. It leads to a theoretical diameter of 3.8 nm, which is close to the experimentally estimated diameter. A 3.8 nm diameter spherical gold nanoparticle has a surface area of ca 45 nm2. Elemental analysis of LuPc2–AuNP and TbPc2–AuNP showed C:H:N = 7.44%:0.97%:1.44% and 7.11%:0.55%:1.95%, respectively, indicating that the ratios of the complexes and gold atoms are 1:46 and 1:49 for LuPc2 and TbPc2, respectively. Therefore, each gold nanoparticle should have ca 29–30 molecules of LnPc2 on the surface. The number of adsorbed LnPc2 molecules was nearly the same irrespective of the lanthanide ions. In the case of TbPc2–AuNP, a static magnetization measurement provided information on the number of TbPc2 molecules adsorbed on a gold nanoparticle. The χmT value around room temperature was obtained as 12 emu K mol−1 by assuming 30 molecules on each 3.8 nm gold nanoparticle (see Supplementary Fig. S2) which value is in good agreement with that of the free trivalent Tb ion (11.81 emu K mol−1)23. These results give further evidence that the estimated number of molecules adsorbed on one gold nanoparticle and the diameters of nanoparticle are appropriate.

The cross-sectional area of a LnPc2 molecule for the flat-on configurations was estimated to be 1.63 nm2 from a molecular model24 constructed using Mercury (Ver. 2.3) and ImageJ (Ver. 1.43u). The surface area of ca 45 nm2 for the 3.8 nm gold nanoparticle can accommodate 28 molecules of LnPc2 in flat-on configurations, which agrees well with the result of elemental analysis. A schematic representation of LnPc2–AuNP is shown in Figure 1. The poor solubility of the assemblies should be explained by the strong π–π interaction between phthalocyanine ligands paralley absorbed on gold nanoparticles surface.

X-ray photoelectron spectroscopy (XPS) analysis for LuPc2–AuNP and TbPc2–AuNP assemblies also demonstrated (1) the existence of LnPc2 complexes and gold nanoparticles in assemblies and (2) neutral electronic states of the complexes on the gold nanoparticle surface (see Supplementary Discussion S1, Figs S3 and S4 and Tables S1–S3).

AC magnetic properties of TbPc2–AuNP

To characterize the magnetic properties of TbPc2 as a single-molecule magnet in the gold nanoparticle assemblies, alternating current (AC) magnetic susceptibility and magnetization of TbPc2–AuNP were studied. Figure 3a shows the temperature-dependent AC susceptibility of TbPc2–AuNP assemblies with various AC-field frequencies. As the temperature decreased, χM′T decreased abruptly, where χM′ is the real part of the AC susceptibility, clearly indicating the phase lag of the magnetization response from the external magnetic field. Each peak position of the χM″/χM vs. T plot, where χM″ and χM are the imaginary part of the AC susceptibility and DC susceptibility, respectively, provides the precise temperature where the χM′T decrease is observed. Ishikawa et al.25 reported that TbPc2 follows the Orbach process, which gives a barrier height (Δ) in a specific temperature range, where Δ is the energy to invert the magnetic moment of Tb3+. Because Δ depends on the strength of the ligand field, we can estimate the oxidation state of the Pc ring from Δ. The value of Δ was estimated from the slope of the Arrhenius plot of lnτ−1 vs. 1/T,

where τ is the magnetization relaxation time calculated from the angular frequency ω of the applied AC field25 and kB is the Boltzmann constant. In Figure 3b, linear behaviour was observed above 40 K, indicating that the Orbach process was dominant. The estimated barrier height Δ was 472 cm−1, which is close to that of the neutral state (Δ = 26025, 41026, 55027 cm−1 for monoanionic, neutral and monocationic TbPc2, respectively). The blocking temperature (TB) corresponding to an anisotropy barrier was about 47 K at 1042 Hz, which is also close to the neutral state (TB = 4025, 5026, 5227 K at 997 Hz for monoanionic, neutral and monocationic TbPc2, respectively). Deviation from the Orbach process at temperatures lower than 40 K is an intrinsic TbPc2 property25 and did not originate from interaction between TbPc2 molecules and the gold nanoparticle.

Figure 3
figure 3

AC magnetic properties of TbPc2–AuNP.

(a) Plots of temperature and AC frequency dependence of χmol′T and χmol″/χmol, where χmol′, χmol″ and χmol are in-phase AC, out-of-phase AC and DC molar magnetic susceptibilities, respectively. The measurements were performed in a 7 Oe AC oscillating magnetic field. (b) Temperature dependence of the magnetization relaxation rate. The solid line denotes linear fits given by equation (1) in the temperature range above 40 K. (c) Magnetic hysteresis circles of bulk TbPc2 and TbPc2–AuNP measured at 1.8 K with an average sweep rate of 10 Oe/s. (d) Cole-Cole plot at 2 K for TbPc2 and TbPc2-AuNP, respectively.

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Figure 3c shows the magnetization curves measured at 1.8 K for bulk [TbPc2]0 and TbPc2–AuNP. No hysteresis behaviour was found for bulk [TbPc2]0, as reported previously25. A clear butterfly-type hysteresis behaviour was observed for TbPc2–AuNP. Such a butterfly-type hysteresis behaviour was observed when TbPc2 was diluted at a concentration of 2% in a diamagnetic matrix (YPc2)25 or attached to single-walled carbon nanotubes through π–π interactions28. Intermolecular interactions between TbPc2 molecules were reduced in TbPc2–AuNP, as with the diluted [TbPc2]0 in a YPc2 matrix.

The Cole-Cole plot for bulk TbPc2 (Fig. 3d), measured at 2 K, shows a single semicircle shape. On the other hand, the Cole-Cole plot for TbPc2-AuNP shows two relaxation phases corresponding to the high and low-frequency regions of applied AC frequency. These results strongly suggest that the environment of a TbPc2 molecule in TbPc2-AuNP is different from the environment in bulk TbPc2. Based on of the opened hysteresis loops (Fig. 3c right) and the Cole-Cole plots, we concluded that the TbPc2 molecules absorbed on the gold nanoparticles in TbPc2-AuNP were relatively isolated from each other.

Electron-transport properties of LuPc2–AuNP and TbPc2–AuNP assemblies

The weak interaction between a gold nanoparticle and the complexes strongly affected the electron-transport properties. The resistance of LuPc2–AuNP at room temperature (RR.T.) was smaller than that of TbPc2–AuNP by one order of magnitude (Table 1), which originates from the difference in the sample volume. The sample volume will determine the number of electron paths described in terms of percolation29 which affects the absolute resistance value, but will not affect the conduction mechanism. Figure 4 shows the temperature-dependent normalized resistance (R/RR.T.) of the assembled structures of LuPc2–AuNP and TbPc2–AuNP. The R/RR.T. of both assembly structures from room temperature to approximately 220 K obeyed an Arrhenius-type temperature dependence (see Supplementary Fig. S5). The activation energies of the nanoparticle assemblies were almost the same, irrespective of the adsorbed molecules; 39.8 meV and 41.8 meV for LuPc2–AuNP and TbPc2–AuNP, respectively. The activation energy is strongly influenced by the particle size and distance between the nanoparticles30. The similar activation energies for the LnPc2–AuNPs are in good agreement with the TEM measurements and elemental analysis, where the monodispersed nanoparticles are separated by LnPc2 in a flat-on conformation in both LuPc2– and TbPc2–AuNP.

Table 1 Room temperature resistance and fit parameters for TbPc2–AuNP and LuPc2–AuNP obtained from T−1 and T−1/2 fits using Arrhenius and ES-VRH models, respectively
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Figure 4
figure 4

Normalized resistance of LuPc2–AuNP and TbPc2–AuNP assembly structures plotted vs. T−1 and T−1/2.

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On lowering the temperature below 220 K, the R/RR.T. deviated from an Arrhenius plot. The values of R/RR.T. became proportional to exp(T−1/2) in the temperature range from 82–15 K and 123–20 K for TbPc2–AuNP and LuPc2–AuNP, respectively (see Table 1 and Supplementary Fig. S6). Below 20 K, R/RR.T. of LuPc2–AuNP became temperature independent down to 1.8 K, while that of TbPc2–AuNP continued to obey R/RR.T. ∝ exp(T−1/2).

The semiconducting charge transport behaviours in nanoparticle assemblies reported so far often obeyed the variable-range hopping (VRH) model,

where T0 is a constant and the value of γ ranges from 0 to 1. γ = 1 represents Arrhenius behaviour. If strong Coulomb interactions exist, then the system obeys a γ = 1/2 law, which is independent of the dimensionality and is known as ES-VRH31. Therefore, we consider that R/RR.T. in the temperature range below 220 K should obey ES-VRH-type behaviour.

Here, ToES represents

where kB is the Boltzmann constant, e is the electron charge, ε0 is the dielectric constant in a vacuum, εr is the relative dielectric constant of the surrounding medium (εr = 30 and 28 for LuPc2 and TbPc2, respectively, obtained from dielectric measurements) and a is the localization length of the electronic wave functions. We evaluated the electronic coupling strength between gold nanoparticles using the localization length a estimated from the specific value of ToES. The coefficient β depends on the dimensionality of the conducting electrons (for two and three dimensions, β = 6.5 and 2.8, respectively)32. We used 6.5 for β because we found that the two-dimensional pathway was used from analysis of the I–V characteristics, as mentioned below. Table 1 summarizes the parameters of the ES-VRH models. From equations (3) and (4), we obtained a = 1.18 nm and 0.71 nm for LuPc2–AuNP and TbPc2–AuNP, respectively, which were smaller than the diameter of the gold nanoparticles. A consideration of weakly protected gold nanoparticle feature based on the localization length a will be described in discussion section.

The values of the hopping distance (rhop)30,33 of the conduction electrons can be estimated from

where κ is the tunnelling decay constant, which is the reciprocal of the localization length a (κ = a−1). The rhop values are summarized in Table 1. Those for both LuPc2–AuNP and TbPc2–AuNP were equal at 1.5 nm, well matched with the nearest-neighbour particle distance, at the temperatures where the transport mechanism crossed over from an Arrhenius-type thermally activated hopping to an ES-VRH-type hopping mechanism around 123 K and 82 K for LuPc2–AuNP and TbPc2–AuNP, respectively. The results indicate that the cross-over from Arrhenius- to ES-VRH-type conduction was decided by the rhop distance of the transport electrons.

The I–V characteristics of the LuPc2–AuNP below 20 K (Fig. 5) were explained by the model of Middleton and Wingreen (the MW model)34, which is based on electron transfer through the nanoparticle assembly taking into account randomly charged nanoparticles separated by tunnel junctions. The I–V characteristics of the model are described by

where VT is the Coulomb-blockade threshold voltage and ξ is the dimensionality of the current pathway, which depends on the structure. The values of ξ = 1 and 5/3 were calculated for infinite one-dimensional and two-dimensional arrays, respectively and numerical studies indicated that 2.2<ξ<3.0 for multilayered arrays35,36. The model reproduced the I–V characteristics of LuPc2–AuNP at 10 K well (Fig. 5a), indicating that electrons tunnel through the nanoparticles with the Coulomb blockade. The value of ξ remained approximately 2.5 irrespective of the temperature from 1.8 K to 10 K (Fig. 5b). This indicates that the dimensionality of the tunnelling electron flow is not strictly two-dimensional because the LuPc2–AuNP structure is composed of the random arrangements of nanoparticles demonstrated by TEM measurements (Fig. 2). The reasonable value of ξ implies that the contribution from the ES-VRH-type hopping mechanism was completely suppressed. The threshold voltage below 10 K was in the range 0.4–0.6 V and decreased linearly with increasing temperature at 20 K keeping the same ξ value. This implies that the tunnelling electrons were under competition from the thermal fluctuation and the Coulomb effect below 20 K. The reason that the threshold voltage changed and ξ remained unchanged with increasing temperature above 10 K is because the thermal fluctuation disturbed the Coulomb effect while the dimensionality of the electron flow was maintained37.

Figure 5
figure 5

(a) I–V characteristics of a LuPc2–AuNP assembly at 10 K. The red line denotes the MW model given by equation (3). The parameters VT and ξ were 0.48 V and 2.4, respectively. (b) Temperature dependence of the parameters VT and ξ.

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Discussion

We consider van der Waals interactions to be the predominant interaction between LnPc2 and the surface of the gold nanoparticles for the following reason: XPS analysis showed the molecular structure of both TbPc2 and LuPc2 phthalocyanine ligands to be unchanged before and after stabilization of the gold nanoparticle. Additionally, the neutral electronic state was confirmed by the blocking temperature (TB) value, which was estimated by the AC magnetic properties. Komeda et al.38 concluded through STS measurements that neutral TbPc2, with an unpaired electron in the π-orbitals of the phthalocyanine ligand, interacted weakly with a flat Au (111) substrate surface. Based on the XPS and AC measurement results, v. d. Waals interactions are the dominant interactions between the lanthanide complex and the gold nanoparticles.

Weakly protected gold nanoparticle by LnPc2 through v. d. Waals interactions showed a small localization length a and the I-V characteristics reproduced by a MW model for electron transport measurements. The localization length a demonstrates the weak interaction between a gold nanoparticle and the complexes. In nanoparticle arrays, the magnitude of a should be equivalent to the size of each nanoparticle39,40 and the larger values of a imply a stronger coupling between nanoparticles. For example, Romero et al.41 reported the localization length a of 3.5 nm, which is comparable to the diameter for oleic acid-ligated PbSe nanoparticles (2.75 nm), which increased in size to approximately 102 nm after vacuum annealing, where the diameter then remained constant. They suggested that the origin of the large a was due to strong coupling between PbSe nanoparticles through reduction of the barrier heights by annealing. In LuPc2–AuNP and TbPc2–AuNP, each nanoparticle packed densely in the assembled structure and the internanoparticle distance is short (~1 nm), as seen from the TEM images (Fig. 2). Despite that, the localization lengths a for LuPc2–AuNP (1.18 nm) and TbPc2–AuNP (0.71 nm) were significantly smaller than the particle diameter (about 4 nm). This strongly suggests that the gold nanoparticles are in a weak electronic coupling state with each other because both of the LnPc2 molecules adsorbed on its surface through weak v. d. Waals interactions.

The I–V characteristics of the LuPc2–AuNP showed the non-liner behaviour with threshold voltages described by the MW model. If the electronic coupling between the nanoparticles became strong, such as systems with covalent bonds between the gold nanoparticles and a saturated40 or conjugated molecule42, the conduction behaviour tends to obey a coherent tunnelling model. Coherent tunnelling provides a conduction channel at low applied bias through several junctions resulting in the power law I–V behaviour with no threshold voltage40. The I–V characteristics obeyed the MW model closely, indicating the weak electronic coupling in the LuPc2–AuNP assembly.

Such a weak v. d. Waals interaction prevents the strong hybridization between the molecular orbitals of the complex and the Fermi level of gold38. We confirmed the small localization length and threshold voltage behavior by electron transport measurements. These results demonstrate that the transport electrons are confined within the nanoparticles because the complexes adsorbed on the nanoparticles act as an insulating barrier to prevent the electronic coupling between adjacent nanoparticles. This means that the electronic structures of the “gold nanoparticle-complex-gold nanoparticle” system were isolated from each other. Taking these results into account, the interaction between the LuPc2–AuNP and the gold surface is considered weak.

In contrast, the TbPc2–AuNP assembly did not show a temperature-independent behaviour of R/RR.T. down to the lowest temperature (Fig. 4). Additionally, the I–V characteristics did not obeyed the MW model (see Supplementary Fig. S7). As for the electron transport through TbPc2, the Kondo effect, which originates from the correlation between the π-spin of the phthalocyaninato (Pc) ligand and the tunnelling electron, has been reported by Komeda et al.38. Vincent et al.43 utilized the Kondo effect derived from the Pc ligand to detect the quantum tunnelling of the magnetization (QTM) of a single Tb3+ ion. Both examples suggest that the direct interaction between the transport electron and the Tb3+ electronic state did not occur. The electronic state in both LuPc2–AuNP and TbPc2–AuNP is neutral. Therefore, they have a π-spin on the phthalocyaninato ligand. Because similar activation energies were observed above 20 K for both LuPc2–AuNP and TbPc2–AuNP, the difference between the conduction behaviors at lower temperature could not be attributed to the difference in the number of conduction paths between electrodes or the difference in sample dimensions. One of the possible causes of this result is a direct correlation between the tunnelling electron and the spin states of the Tb3+ ion, although this requires needs further investigation.

In conclusion, we synthesized and characterized gold nanoparticle assemblies protected by bis(phthalocyaninato)lanthanide(III) complexes, which were stabilized through v. d. Waals interaction between the π-conjugated macrocycle of the phthalocyanine ligand and the gold nanoparticle surface. The assembly structures were composed of monodispersed gold nanoparticles with a diameter of about 4 nm. The phthalocyanines were adsorbed with a parallel conformation on the gold surface and both the LnPc2 and the gold nanoparticles were in the neutral states. TbPc2 showed the magnetic properties of single-molecule magnets with a clear hysteresis loop in TbPc2–AuNP, showing that the molecules were isolated from each other on the gold surface. Even though LnPc2 did not have a specific binding unit capable of covalent bonding or electrostatic interaction, it stabilized the gold nanoparticles through v. d. Waals interaction between the π-conjugated macrocycle of the phthalocyanine ligand and the gold nanoparticle surface. The weak interaction between the gold nanoparticle and the phthalocyanines affected the electron-transport properties. The electron-transport mechanism for both assembled structures changed depending on the temperature. By decreasing the temperature from R.T. to 1.8 K, they showed an Arrhenius-type thermally activated hopping, ES-VRH-type hopping and collective single-electron tunnelling. The geometrical equivalences between the LuPc2–AuNP and TbPc2–AuNP assembled structures were demonstrated by the activation energy and hopping distance obtained from the Arrhenius model and ES-VRH model, respectively. The localization length a of the transport electron in the ES-VRH model was significantly smaller than the diameter of the gold nanoparticles, indicating weak electronic coupling between the gold nanoparticles in LnPc2–AuNP. Collective single-electron tunnelling was observed in the LuPc2–AuNP assembled structure, indicating that the electrons were confined within each gold nanoparticle because of weak electronic coupling between gold nanoparticles at lower temperatures. Such a Coulomb-blockade feature will collaborate with the emerging field of bioelectronics, for example, the biogating behaviour of coupling with living cells44 and/or the stochastic resonance to construct artificial neuron communication45.

Methods

Synthesis of LuPc2–AuNP and TbPc2–AuNP assemblies

The neutral complexes of LuPc2 and TbPc2 were synthesized according to the literature46. Gold nanoparticles protected by LuPc2 or TbPc2 were synthesized using a two-phase system, as described by Brust et al.47. A 10 ml aqueous solution of HAuCl4·3H2O (35 mg, 0.09 mmol) was mixed with 30 ml toluene solution of tetraoctylammonium bromide (TOAB). The two phases were stirred until all of the HAuCl4 had transferred into the toluene phase. A 30 ml toluene solution of LuPc2 or TbPc2 (104 mg, 0.09 mmol) was added and the mixture was stirred for 15 min at 10°C. Then, 1 ml aqueous solution of NaBH4 (33 mg, 0.3 mmol) was slowly added. After stirring for a further 10 min, the toluene phase was separated and concentrated. Ethanol was added and the resulting precipitate was washed with ethanol, toluene and chloroform to obtain gold nanoparticles protected by LuPc2 or TbPc2 (referred to as LuPc2–AuNP and TbPc2–AuNP, respectively) (Fig. 1).

Materials characterizations

TEM measurements were carried out using a Hitachi HD-2000 TEM employing an acceleration voltage of 200 kV. Samples for the TEM measurements were prepared on a carbon-coated Cu grid (Okenshoji Co., Ltd., Japan) by drop-casting of a toluene solution of the LuPc2–AuNP and TbPc2–AuNP suspension and then the toluene was allowed to evaporate. The XPS measurements were carried out using a JPS-9200 electron spectrometer (JEOL, Japan) equipped with a rotating anode Al Kα X-ray source. The pressure of residual gases in the analyser chamber during XP spectra acquisition was 4×10−6 Pa. The peak positions were calibrated using the binding energy of Au (4f7/2 = 84.0 eV) and In (3d5/2 = 433.9 eV). The spectra were fitted by symmetric Voigt functions after subtracting the Shirley-type background. The temperature-dependent AC and DC magnetic susceptibility, hysteresis loop measurements and Cole-Cole plot were carried out using a Quantum Design MPMS-XL5 magnetometer for a TbPc2–AuNP powder sample. For electron-transport measurements, toluene suspended solutions of TbPc2–AuNP or LuPc2–AuNP were drop-cast onto gold electrodes (thickness of 50 nm and a gap of 30 μm) formed on SiO2 substrates. After the solvent was evaporated in a vacuum at room temperature, temperature-dependent DC resistance and current–voltage (I–V) characteristics were measured using a picoammeter voltage source (Keithley Model 6487) with a constant bias voltage in the range of −5 to +5 V in a PPMS (Quantum Design Japan) over the range 300–1.8 K. Measurements were carried out on at least five samples to confirm the reproducibility.