Molecular Anchoring with 4-Mercaptobenzoic Acid and 4-Aminothiophenol for Using Active Nanorods in the Detection of Dopamine

The incorporation and effective anchorage of gold nanorods in a gold (111) substrate is applied to electrochemical systems to detect dopamine. Gold nanorods (AuNRs) were synthesized in dispersion. They were then incorporated in a metal substrate mediated by self-assembled monolayers (SAMs) which act as structural anchors. Two molecular anchors, 4-mercaptobenzoic acid (4-MBA) and 4 aminothiophenol (4-ATP) are compared by means of the charge density (Q) in desorption of the SAMs, where 4-MBA presented a greater coverage on the metal surface. Both SAMs allowed the effective confinement and communication of the nanostructure to a greater or lesser extent. Characterizations were made to confirm the constructed system. First, the nanostructures synthesized in dispersion were characterized by UV-visible spectroscopy, Transmission electron microscopy and atomic force microscopy. Second, an electrochemical characterization of the working electrodes include impedance was made. The results focus on the impact of the molecular anchor on the activity of the electrochemical sensor, it was determined. Reducing the charge transfer resistance (by at least 90% with appropriate SAMs) of molecularly anchored gold nanorods increases the sensitivity of the electrochemical sensor (at least 20%), the detection of dopamine was studied by square wave voltammetry through a calibration curve, where better sensitivity and detection limit was obtained with the Au/4-MBA/AuNRs system compared to Au/4-ATP/AuNRs.

Nanosystems can show interesting physical behaviors. If these nanosystems are incorporated in substrates, they are potentially useful for the detection of analytes. They are nanostructured substrates which under given experimental conditions present behaviors based on quantum phenomena (electron confinement, superficial plasmon effects, 1 among others). In recent years the manufacture and synthesis of nanostructured materials, specifically gold nanostructures with controlled size and shape, has become simpler, in stable monodisperse colloidal systems.
Nowadays it is possible to control the sizes between 1 nm and 150 nm and obtain a closely controlled morphological distribution. As reported by, 2 gold nanorods (AuNRs) in particular present high reactivity, due to the crystallographic index differences, which have a high content of (100) orientation, and due to its anisotropy. However, the main basis of these nanosystems is doubtlessly the anchorage, incorporation, and effective confinement of nanostructures to an electrode substrate, the factor that finally allows developing the range of properties of the nanostructures.
The anchor molecules must be based on self-assembled monolayer systems (SAMs), 3 in which three parts can be distinguished: a thiol group which acts as a terminal part or anchorage group to the support. In the case of gold surfaces, the sulfur-gold interaction allows its covalent anchorage on the surface, an organic skeleton or spacer group that stabilizes the structure by means of weak Van der Waals type interactions, and/or π interactions (if there are aromatic structures) with the neighboring chemisorbed skeletons and a head or functional group that is exposed to the environment or interface. The spontaneous formation of these organic monolayers on solid surfaces was shown for the first time in 1946 by Bigelow et al. in the case of the alkylamines on Pt. 4 Self-assembled monolayers (SAMs) are formed spontaneously from the vapor phase solution combining the advantages of ultrahigh vacuum (UHV) and the Langmuir-Blodgett method. 5,6 As in the case of films formed under UHV, the method is simple and there is a strong interaction between adsorbate and substrate, because the molecules are chemisorbed. Because of the strong interaction between the alcanethiol molecule and the Au(111) substrate, the S head is chemisorbed in specific sites of the substrate, in this way forming commensurate networks in the Au(111) mesh network. On the other hand, as in the Langmuir-Blodgett films, the chain-chain interactions (which involve van der Waals forces, π, and hydrophobic interactions) ensure an efficient packing of the monolayer. 7 Nowadays it is accepted that when aromatic thiols are adsorbed on the Au(111) surface build up the well known 22 × √3 reconstruction of Au(111) and form RS-Au ad -RS complexes (where Au ad are Au adatoms) called "staples." 8,9 However, the chemical interaction is still under discussion because the influence of different types of intermolecular forces (van der Waals, π, and "soft-soft") is analyzed and the oxidation of the adatoms. 10,11 Due to the formation of these complexes which involve Au adatoms, isles of vacancies or "pits" are produced in the Au(111) terraces.
On the other hand, SAMs of different aromatic thiols have been studied (with different numbers of aromatic rings) on Au(111), which also put up the 22 × √3 reconstruction, but present some discrepancies with respect to those of alkanethiols: they do not induce the formation of "pits" on the surface, but on the contrary, sometimes generate isles of Au atoms. 12 Furthermore, these monolayers are very stable once prepared, they are not sensitive to moisture, and do not polymerize, in contrast to the SAMs of silanes, which are hydrolyzed and form polymers that can contaminate the surface. However, various factors affect the quality of the thiolate SAM, in this case of the SAMs formed in solution, we can mention the crystallinity and the roughness of the gold substrate, the nature of the adsorbate (the hydrocarbon chain length, the functionality of the terminal group, etc.), the temperature, the solvent used, the incubation time, and the adsorbate concentration, among others. 13 Another important factor that must be considered in nanostructured systems is the way of electron transfer (ET), which can be defined according to the chain length of the molecular anchor; for short molecules, a rapid exponential decay of the current with distance, not depending on the temperature, ET by tunneling take place. On the contrary, if the molecular anchor corresponds to long molecules it will present a slow drop of the current with distance and a marked dependence on the temperature, the ET Hopping process. 14 Because of the factors mentioned above, the idea of a "best" selfassembled monolayer is far from being a reality, due to the existence of different types of defects. Taking this premise into account, short z E-mail: sara.ramirez@usach.cl; juan.silva.r@usach.cl chain thiols like 4-mercaptobenzoic acid (4-MBA) and 4-aminothiophenol (4-ATP) were studied. They have the property of forming self-assembled monolayers that allow effective communication between nanostructures and metal surfaces. These molecules correspond to molecular linkers which increase both electron transport and transfer in nanostructured systems, in this way improving the activity and sensitivity of the electron surfaces for the detection of dopamine in nanostructured systems.
Dopamine (DA), on the other hand, is an important neurotransmitter in biological systems. It is distributed extensively in brain tissues and participates in the transfer of messages in the central nervous system. DA defficiency can cause some neurological disorders such as schizophrenia and Parkinson's disease. 15 Because of these reasons, the design of new nanoelectrodes based on the effective incorporation of AuNRs to metal substrates by means of molecular 4-MBA and 4-ATP linkers would allow solving fundamental information on the process of detection of neurotransmitters (Scheme 1), in this way increasing electron transport and transfer, reflected in the improvement of electrochemical activity and sensitivity of the electron surfaces. 16,17 For other side, the internet of things is a new revolution that will shift people's lifestyles to a high-tech. IoT refers to the process of connecting devices via the Web, including wearables, power gadgets, mobiles phones, and biosensors to work automatically and semiautomatically to execute many functions and gather data to enhance productivity and accuracy. Smart cities, smart homes, pollution management, energy conservation, smart transportation, smart industries, and healthcare are examples of IoT-driven developments. 18,19 This last topic includes detection, treatment, and monitoring of diseases. The IoT in healthcare could improve patient care due to various smart devices like ultrasound, mobile Xray machines, smart beds, glucose meters, blood-pressure analyzers, and thermometers units. 20 Portable, biocompatible, fast sensing, compact, easy to operate, stable, sensitive, etc. are some characteristics of sensors. Added, to the fact that in the future the connection with other devices via Bluetooth or internet could be develop, integrate IoT sensors possesses immense potential in designing e-healthcare with intelligent, flexible, and interoperable operations. 21 The use of gold nanorods in IoT sensors has demonstrate interesting results, such as SARS-CoV-2 detection, 22 human pulse analysis 23 and melamine detection. Therefore, this work could contribute to IoT neurotransmitters detection sensors development that allow constant monitoring of these substances. In this investigation the key design of the interface is provided for correct used of self-assembled monolayer, because this molecular anchoring system could modify the activity of the sensor of dopamine with great potential application on integrate IoT sensor.

Methodology
Synthesis of AuNRs.-The synthesis of AuNRs was made as described by por Nikoobakht et al. 2,24 The AuNRs were obtained through a process of two crucial steps.
The first one is the formation of the seed. In a vial 9.5 ml of 0.1 M cetyltrimethylammonium bromide surfactant (CTAB) (Sigma) and 250 μl of a 10 mM solution of HAuCl 4 (Sigma Aldrich), with constant stirring. Then 600 μl of a cold 0.01 M solution of sodium borohydride (NaBH 4 ) (Sigma Aldrich) was added, causing an immediate color change from yellow to brown, stirring during 30 min. Finally, the seed solution was allowed to stand for 2 h at a temperature of 30°C.
The second was the preparation of the growth solution. In a stirred vial the reagents were added in the following sequence: 9.5 ml of 0.1 M CTAB, 75 μl of 0.01 M silver nitrate (AgNO 3 ) for well-known preferential adsorption on Gold faces (100) which allows the growth of rods 2,24 (Sigma Aldrich), 500 μl of 0.01 M HAuCl 4 , 55 μl of the reducing agent, 0.1 M ascorbic acid (Sigma Aldrich). A color change from yellow to colorless indicated the reduction of the gold salt in the solution. Finally, 250 μl of 0.1 M hydrochloric acid (HCl) and 12 μl of the seed solution obtained in the first stage of the synthesis was added. The reaction was allowed to proceed for 10 min without stirring, at 27°C, and it was then centrifuged at 13000 rpm during 15 min. Finally, the precipitate was separated from the supernatant and it was suspended in 4 ml of Milli-Q water.
Characterization of gold nanorods (AuNRs) in solution.-a. UV-vis spectroscopy: In a Cary 1E double beam spectrophotometer with a Varian fixed band width of 1 nm, and 1 cm quartz cells, the optical properties of the gold nanostructures were studied by means of the plasmonic absorption bands characteristic of each structure, 25 Transmission Electron Microscopy (TEM): A JEM 2100 HT Transmission Electron Microscope was used.
Modification of the working electrodes.-The gold substrate (plate) consists of an evaporated layer of polycrystalline gold 250 ± 50 nm thick, on a polished support corresponding to a glass sheet 0.7 ± 0.1 mm and a 2.5 ± 1.5 nm chromium layer that facilitates the adhesion. The plate has is 12 ± 0.2 mm long and 12 ± 0.2 mm wide (Arrandee®, Germany). A minimum of three plates were prepared for each medium (n = 3).  electrodes and gases. As working electrode use was made of an already mentioned gold substrate (plate), a silver/silver chloride (Ag/AgCl) as reference electrode, and a Pt wire with a surface area as auxiliary electrode. Electrochemical impedance spectroscopy (EIS) studies were carried out at room temperatura using a PGSTAT302N Metrohm Autolab (NL) potentiostat with a FRA32M module. The EIS measurements were executed at an open circuit potential (OCP) with a 5 mV (peak-to-peak) AC voltage in a frequency ranging from 0.1 MHz to 100 mHz in 2.5 mM [Fe(CN) 6 ] 3− / 4− , each in 0.5 M KCl.
Characterization of the gold substrate, modified electrodes.-By means of a cyclic voltammetry (CV) using 0.1 M sulfuric acid (H 2 SO 4 ), and with a medium saturated with N 2 , the electrochemically active area of the gold substrate was obtained.
Using the same technique, but in a 0.1 M sodium hydroxide (KOH) medium, the potential vs Ag/AgCl from −0.6 to 0.6 V was used for the determination of the crystallographic orientation of the gold substrate.
In the same way described above, with 0.1 M sodium hydroxide (NaOH), but with the self-assembled monolayers present in the gold substrate, the chemical desorption of the SAMs (4-ATP and 4-MBA) was determined, using the potential vs Ag/AgCl from 0.2 to −1.

Results and Discussion
Characterization of the gold nanostructures (AuNPs and AuNRs) in solution.-UV-vis spectroscopy, TEM.-Spectroscopy is a very useful tool for the characterization of nanoparticles, due to the optical properties of nanostructured materials with respect to the macroscopic scale. Metal nanoparticles can present a surface resonance plasmon with an absorption maximum at a specific wavelength that characterizes it, a property which is dependent on the nature of the metal as well as on the size, shape, and interactions between the particles. 26 The UV-vis spectrum of Fig. 1A, corresponding to the AuNRs, shows two plasmonic absorption bands. One at 520 nm corresponds to the transverse resonance of the nanorods, and the other one, at 720 nm, which corresponds to the longitudinal section, has a high absorbance compared to the transverse. This is based on the fact that AuNRs present an aspect ratio greater than 1 (length/width), because the plasmonic bands are directly related to the number of electrons on the surface and the longitudinal section has a greater surface, so it has a more significant number of electrons on the surface and this results in greater absorbance. Figure 1B shows the TEM images and the corresponding histogram (Fig. 1C) of the AuNRs stabilized in CTAB. A homogeneous size and shape distribution is seen. The histogram obtained from a population of 100 particles gives an aspect ratio centered on the value of 4.5 ± 0.9, with a length of 38 nm and a width of 8.5 nm.
The results of transmission microscopy agree with those of the UV-VIS spectroscopy for the AuNRs. Both show the anisotropy of the nanostructure, producing an aspect ratio greater than 1 consistent with the rods structure.
Electrochemical characterization of the working electrodes (gold).-The electrochemical characterization of crystallographically oriented metal surfaces, can be made because their superficial redox processes reveal their defined and particular structural characteristics.
Electrochemical characterization of the Au/SAMs in basic medium.-For the effective electron incorporation and communication of the gold nanorods with the metal surface it is es necessary for the electrode to be modified with the SAMs of 4-MBA or 4-ATP. This modification took place by the adsorption mechanism of the thiolated molecules, in which the thiol loses the H atom of hydrosulfide (−SH), getting a negative charge and anchoring to the surface as thiolate by orienting the el terminal S atom to the surface of Au(111). This kind of anchorage can respond to make the aromatic thiols be adsorbed on the surface of Au(111) putting up the well known 22 × √3 reconstruction of Au(111) and forming Au ad -SR complexes (where Au ad are Au adatoms called staples. 8 Also is accepted that a given number of gold atoms are extracted from the upper one (or two) layers of the gold surface to form the gold-thiolate complex, leaving monoatomic vacancies that produce bigger vacant isles by Ostwald maturation.
The sulfur-gold bond that retains 4-ATP and 4-MBA in place is strong (∼44 kcal mol −1 ), 27 so the chemical adsorption is stable and robust. To confirm the formation of the SAMs on the Au surface, a VC was carried out in the cathodic sense to cause the breakage of the Au-S bond due to the presence of the thiol on the surface. Figure 2 shows the voltammograms of the SAMs (4-MBA and 4-ATP), which correspond to its reductive desorption from the Au surface at −0.614 V for 4-MBA and −0.640 V for 4-ATP scanning in the cathodic direction. A Faradic process corresponding to the reduction peak of the S-Au bond can be seen. 28 This process takes place due to the reductive desorption of the thiol anchored to the Au surface according to Eq. 1. In both cases a second small peak is also seen, which can be attributed to the desorption of the SAMs on crystallographic faces different from Au(111).  Table I shows the charge density of each SAM, yielding greater values compared to those in the literature. 28 This improved yield is due to our effective incorporation technique based on a low concentration and an extended incubation period. However, the procedures found in the literature saturate the electrode with a high SAM concentration and short incubation periods.
Taking these charge density values into account it is possible to determine the surface coverage of thiolates (θ RS ) 0.35 and 0.27 for 4-MBA and 4-ATP, respectively. This surface coverage relates perfectly with the 22 × √3 construction of Au(111), which shows a high thermodynamic stability in terms of the free energy calculated by DFT. 28 A greater charge density stability is shown by 4-MBA, and therefore a greater surface coveage of thiolates compared to 4-ATF. This behavior can be explained by the presence of a greater packing of the 4-MBA SAMs due to a small energy barrier that arises from the π interactions between adjacent molecules, and it could be the reason for finding molecule domains in sites, holes, or foci centered on the (fcc) faces, which correspond to the more stable forms. It can also be due to the different hydrogen bridges possibly generated in the formation of the monolayers. 29 Electrochemical characterization of the Au(111)/SAMs system modified with AuNRs, with a redox potassium ferri/ferrocyanide probe.-CV is an effective method that enables monitoring of the electrochemical behavior of the electrode after each step of modification. In addition, CV allows the characterization of the interfacial analytical processes through changes in peak currents that are related to electron transfer resistance. 30,31 Figure 3 shows the voltammograms of the Au(111) systems (black line), Au/SAMs (4-MBA, 4-ATP) (red line), and Au/SAMs/ AuNRs (green line). First, it is seen that modifying the gold surface with the SAMs there is a decrease of the current and a greater separation of the anodic and cathodic peak potentials, responding in this way to a system with greater irreversibility. In this context it is known that in the moderately active SAMs the electron transfer takes place by a superexchange, or tunneling, mechanism through the bond. This behavior is due to the fact that the electron transfer takes place through the molecular bridge, with vacant energy levels higher than the energy of the electrons that is transferred above the Fermi level of the donor. Furthermore, the electron transfer takes place in one step and at a rate that drops exponentially with distance. Since 4-MBA and 4-ATP are short molecules, the electron that is transferred does not reside directly on the molecular bridge, and the states of the states of the molecular bridge during the time that goes by since the electron leaves the donor until it is localized in the acceptor is known as virtual excitation.
Second, incorporating the AuNRs to the Au/SAMs (4-MBA and 4-ATP) systems there is a considerable current restoration, together with an increase in the degree of reversibility of the system. This behavior is attributed directly to the action of the nanorod, which presents a greater electrochemically effective area compared to the gold electrode, linking better in the Au/4-MBA/AuNRs system due to the strong interaction of the head carboxylate group in the face of the gold nanostructure, additionally is joined by the high charge density (Q) of the self-assembled monolayer, which can interact with a greater number of AuNRs. However, in the case of the 4-MBA/4-ATP system, although the current caused by the effect of the AuNRs is restored, a lower degree of reversibility is seen. This behavior can be attributed to the low interaction of the head group with the gold of the nanorod, which would indicate a smaller number of AuNRs anchored on the surface.
Electrochemical impedance, for its part, is a technological tool that provides additional information on the kinetics of electron transfer between the probe and the electrode surface. Charge transfer resistance (R CT ) corresponds to diameter of the half circle of the Nyquist diagram and is an effective parameter to monitor the interfacial process. 32 Figures 3C and 3D show the impedance spectra for the assembly steps of the nanostructures, obtained in a frequency range between 100 Hz and 1000 Hz. The electrochemical characterization of the Au (111) electrode, Figs. 3C and 3D black line, showed a low impedimetric response, Table II (R CT = 56.5 Ω) represented by a Nyquist diagram with a small semicircle and a quasi-linear behavior.
After the adsorption of the different SAMs, 4MBA (Fig. 3C red line) and 4ATP (Fig. 3D red line), an increase in the resistivity of the system was verified for 4MBA (Rct = 330 Ω) this may be due to the partial blocking of charge transfer from electrical double layer, indicating proper formation of the monolayer. This partial blockage may be due, on the one hand, to the carboxyl group (-COO-) that removes the electronic density of the system and causes less interaction between the sulfur and gold atoms. In this case no adatoms would be formed. However, as a whole, these 4MBA molecules generate an associative effect between them, through the interactions of neighboring chemisorbed molecules, generating compact monolayers with a more significant surface coating (Fig. 2). On the other hand, this partial blockage may also be associated to the electrostatic repulsion between the carboxyl group (−COO−) and the negative charges of the electrolytes Fe (CN) 6 3− /Fe CN) 6 4− of the redox process of the electric double layer. For the case of SAMs 4ATP a different behavior is observed, because the value of Rct = 46.3 Ω, Table II, indicates that the electron transfer capacity of the gold electrode modified with 4ATP is improved compared to gold, this behavior can be attributed at electrons flow with the conjugation of the amino group. That is, when a system is correctly conjugated between the sulfur atom and the amino group, the electron transfer speed is improved compared to that of the bare gold electrode. 33 Then, when adding the nanostructures to the systems, an improvement is observed in the electronic transfers, the Au/ 4MBA/AuNRs electrode, Fig. 3C green line, when the AuNRs are   incorporated to the modified 4MBA electrode, a considerable reduction in resistance to the electron transfer (R CT = 6.56 Ω). This result is due to the physicochemical properties of the Au/ 4MBA/AuNRs system that cause an increase in the flow of electrons between the electrode surface and the solution. This increase in the flow of electrons can be explained according to the strong coordination between the head group (−COO−) and the AuNRs, where it is presumed that there is an increase in the energy of the system, reflected in a greater transfer capacity of electrons, by the perfect configuration between the molecular orbitals of the SAMs 4MBA with the fermi level of the electrodes. The other Au/4ATP/AuNRs system, Fig. 3D green line, presents a resistance value (R CT = 116 Ω) greater than that of bare Au and that of Au/4ATP, this behavior may be due to the fact that when the electrode is modified of Au (111) with SAMs 4ATP there is a perfect conjugation between the Au (111) atom of the electrode, the sulfur atom and the amino group of the 4ATP molecule, which is reflected in a good electron transfer, Table II, giving charge density to the system, presuming in this sense the formation of Au adatoms. In this context, when AuNRs are incorporated into the system they do not modify the strong interaction of the possibly formed adatoms. Consequently, the energy of the Au/4ATP/AuNRs system is located almost entirely in the interaction between Au (111) and sulfur.
Characterization of the Au/SAMs/AuNRs system by atomic force microscopy (AFM).- Figure 4 shows the AFM images for the system consisting of Au/SAM/AuNRs. With the SAMs 4-MBA there is a homogeneous incorporation without agglomerations of the AuNRs on the surface. Moreover, the shape and size distribution are clearly seen. In Figs. 4A-4C this anchorage is attributed to the interaction of the head group when faced with the gold nanostructure. In the case of 4-ATP agglomerations are seen on the surface, Figs. 4D-4F.
Detection of DA with the Au/SAMs/AuNRs system.-A Faradaic is seen in the detection of DA in 4-MBA systems (Figs. 5A and 5B), corresponding to the electrochemical oxidation of the analyte according to the Scheme 2. Therefore, detailed scheme showing the sequence of electrode (E) and chemical (C) steps is presented here as Scheme 2. The product of exchange of two electrons (first step E), dopaquinone (DOQ) as a Michael acceptor undergoes nucleophilic attack of amine in the side chain. The intramolecular proton transfer leads to a transient species, also an a,b-unsaturated ketone in equilibrium with the 1,4 dipole form. The latter under influence of water or other base, undergoes an elimination reaction leading to a stable aromatic species, the phenolate anion (step C). Upon protonation of this anion, dihydroxyindoline (DHI) is formed and oxidized further to the aminochrome (AC) (second step E). According to the potential used, the oxidation reduction stage of E1 is observed to determine dopamine.
First, a shift is seen of the 0.5 V oxidation signal when the SAMs of 4-MBA is replaced by 4-ATP. Second, a marked difference is  seen in the shape of the Gaussian peak, with the one corresponding to 4-MBA being sharper. Furthermore, Table III shows the great sensitivity, the low limit of detection, and the limit of quantification of the Au/4-MBA/AuNRs system compared to that of Au/4-ATP/ AuNRs, and results below the expected concentrations were even possible from a DA concentration of 1 μM in the case of the system with 4-MBA, attributed to a more significant number of anchored nanostructures and a high surface arrangement caused by the   interaction with the carbonyl group of the 4-MBA, in this sense indicating a more significant electrochemically active area reflected in the high sensitivities that assume a homogeneous distribution and separated from one another, observed in Figures (A, B, and C) sufficient for each one of them acting electrochemically in an independent manner. In this way, as the AuNRs are sufficiently separated from one another, the planar diffusion with which the molecules reach the electroactive surface is removed, and a radial diffusion regime is installed that minimizes the limitation or the effect of the mass transport on the electron transfer rate. On the contrary, the Au/4-ATP/AuNRs system presented a low degree of nanostructure anchorage (Fig. 4). Therefore, a low sensitivity is seen in the detection of DA (Fig. 5B), which can be attributed to an agglomeration of nanostructures on the surface or a low incorporation of anchored nanostructures, and the lack of interaction sites, in this way minimizing the radial diffusion regime and restoring the planar diffusion.  Therefore, the nanostructured system that used 4-MBA as a molecular anchor presents a greater electrochemical activity, causing a high sensitivity in the detection of DA.
Finally a comparatively results table (Table IV), with the literature [refr] regarding gold nanostructures incorporated into electrode surfaces for dopamine sensors. In the low range of concentrations it is possible to highlight the sensitivity of the anchored nanorods system. This sensitivity is highly dependent on the charge transfer at the interface (a low resistance Fig. 3). Promising results are key to new sensor designs and smart interfaces.

Conclusions
The functionalization of self-assembled monolayers on the surface of Au(111) was confirmed by voltammmetric techniques, specifically cyclic voltammetry, in the desorption of the thiol, obtaining a greater charge density (Q) in the desorption of 4-MBA compared to 4-ATP. That is, the SAMs of 4-MBA formed monolayers with a better surface packing.
The relation of the molecular anchor of 4-MBA and 4-ATP was established with the study of the Fe(III)/(FeII) probe for AuNRs, determining the current intensity and irreversibility of the system as the Au surface was modified, showing that the system modified with 4-MBA had a better packing and and an adequate anchorage of AuNRs compared to the 4-ATP system. In this sense, for the detection of DA, the system with a high sensitivity was Au/4MB/ AuNRs. Therefore, this result shows that the self-assembled monolayer of 4-MBA improves the transport and electron transfer surface, acting as an efficient molecular linker, connecting correctly the nanorods with the gold, reflected in an increase of the activity and sensitivity of the sensor electrode for the detection of DA.