Unveiling Adsorption of Boron Dipyrromethene Conjugated PbS

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A: Kinetics, Dynamics, Photochemistry, and Excited States

Unveiling Adsorption of BDY Conjugated PbS Nanocrystals on Pt Electrode Surface: An Approach Using Electrogenerated Chemiluminescence Spooling Spectra and Multivariate Analysis Saeed Bagheri, Giovanni Valenti, and Mohsen Kompany-Zareh J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b09881 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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The Journal of Physical Chemistry

Unveiling Adsorption of BDY Conjugated PbS Nanocrystals on Pt Electrode Surface: An Approach Using Electrogenerated Chemiluminescence Spooling Spectra and Multivariate Analysis Saeed Bagheria, Giovanni Valentib, Mohsen Kompany-Zareha,c* a

Department of Chemistry, Institute of Advanced Studies in Basic Sciences, Zanjan, 4513766731, Iran b

Department of Chemistry ‘‘G. Ciamician’’, University of Bologna, Via Selmi 2, 40126 Bologna, Italy c Department

of Chemistry, Dalhousie University, Halifax, NS, B3H 4J3, Canada *[email protected]

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Abstract This research focuses on the adsorption and molecular scale communication mechanism of PbSBDY, a nano-hybrid system of nano crystal (NC) and a π-conjugated molecule investigated through the electrogenerated chemiluminescence spooling spectra and multivariate analysis. The results show that the charge transmitted from excited state of BDY+ to the surface states of PbS NCs leads to emission quenching of BDY and emission enhancement of PbS NCs at 986 nm. Also, the essence tendency of unpassivated sulfur atoms on (100) facets of the PbS NCs acts as a force for adsorption of PbS NCs on the surface of Pt electrode. This phenomenon was proved by conjugation of BDY as an ECL active compound to the PbS NCs and multivariate analysis of augmented data in different scan rates. The obtained results from multivariate analysis reveal that adsorption of PbS-BDY and charge transferring from BDY to surface states of PbS NCs are independent

on

scan


rate.

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Introduction Electrogenerated chemiluminescence or electrochemiluminescence (ECL) is a redox induced emission where the excited state is generated after electrochemical stimulus 1, 2. As a combination of electrochemical and spectroscopic methods, ECL possesses several advantages over photoluminescence such as very low background and no scattering of the emitted light due to the absence of excitation light source 3, 4. As a result, ECL has become a powerful analytical technique which has widely been studied and used in many fields including immunochemistry devices 7, tumor markers early detection

8

5, 6,

mobile

and very recently imaging 9. In the quest for ever-

increasing sensitivity, ECL has been coupled with nanotechnology to optimize the electron transfer that induces the excited state formation

10-12.

In this context, dye doped silica nanoparticles

13,

inorganic quantum dots 14, 15, carbon dots 16 and polymer dots 17-19 play crucial role benefited with high ECL emission and stability. In this context, the design and manufacturing of optoelectronic devices and biosensors at the molecular scale require understanding the complicated mechanisms of energy and charge transfer at interfaces of nanohybrids. The quantum size effects as a unique property of nanocrystals (NCs) which presents them as an appealing and promising component of application in photonics, optoelectronics, and sensors. Among the different types of NCs, lead sulfide (PbS) NCs are particularly suitable according to their narrow band gap, large exciton Bohr radii and emission in the NIR region 20-26. By finding ECL phenomena from Silicon NCs by Ding et. al. 14, Bard’s group has become a pioneer in the ECL of NCs investigation. It is commonly known that there are two basic types of states in NCs: the surface and the core

1, 14, 23, 27.

Clearly, in comparison with

photoluminescence (PL) in which core absorbs the photons and generates excitons, ECL depends more sensitively on surface chemistry. This is particularly true for small NCs since a significant



fraction of the atoms is located on the surface 23, 27. Theoretically, in the NCs, the ECL spectrum with the bluest wavelengths (corresponding to the highest energy level) originates from the core 15, 27.

In opposite to bulk solid, the role of the surface increases and eventually becomes dominant with reducing the size. This domination could be considered as a compelling factor to improve technological application; though the existence of small facet size, multiple edges and corner sites cause to intricate a detailed understanding and description. Usually, NCs were prepared along with a set of ligands as capping layer with the function of stabilization, nucleation control and tuning the optical and electronic properties of NCs

28-30.

Hybrid materials composing of NCs and π-

conjugated organic molecules are increasingly noted for optoelectronic and sensing applications 24, 26, 31.

BODIPY (Boron-dipyrromethene) dyes (BDY) have a wide class of supramolecular

compounds and have a broad use as laser dyes 32, fluorescent biological labels 33, photosensitizers 34

and optical devices

35.

These π-conjugated organic molecules could be utilized as a surface

ligand for PbS QDs which enhances their performance in optical and electrochemical features 24, 26.

Chemometrics, as a powerful set of mathematical modeling and computational statistics tool, could be applied to discover the underlying patterns in data. It also helps us gain cognitive access to data and interpret the results in the light of prior chemical knowledge and the laws of physics. There are numerous examples for application of multivariate analysis in different types of data 3638.

Herein, we investigate the ECL study on the hybrid system of PbS NCs and BDY (Scheme 1) and their energy transfer mechanism through the multivariate analysis methods:


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Scheme 1. Structure of PbS-OA and PbS-BDY nano-hybrid system

Experimental Section The details of synthesis procedure, electrochemical assays and ECL data acquisition were reported previously 26. Multivariate Analysis and Modelling MATLAB software was used for multivariate analysis based on the Multivariate curve resolution alternating least squares (MCR-ALS) algorithm. The chemometrics programs for MCRALS were written by authors in MATLAB. MCR-ALS is the most applicable method to solve the mixture analysis problem in chemical data. MCR-ALS considers a bilinear model and decomposes 5 ACS Paragon Plus Environment


the data matrix (D) as a mixed measurement to the pure chemically meaningful profiles (C and A). 𝐃 = 𝐂𝐀T + 𝐄 The algorithm starts with an initial guess of either C or A for iterative optimization through alternative least squares with D. In each iteration, calculation of the C and A matrices or both of them could be along with some kinds of constraints to evoke chemically relevant profiles from mathematically modeling. Excluding negative values in C and A matrices and considering one maximum per components (C or A), named as non-negativity and unimodality, are the common employed constraints in MCR-ALS. The iteration is continued until the relative sum of square for elements in E matrix reach to a minimum value. The more details about the MCR-ALS is explained in supporting information. Results and Discussions PbS-Oleic Acid (PbS-OA) and PbS-BDY nano-hybrid system show strong ECL emission in CH2Cl2 by using tripropyl amine (TPrA) as “oxidative-reduction” sacrificial coreactant. Figure 1 shows the ECL emission maps of PbS-Oleic Acid (PbS-OA) and PbS-BDY during the potential scanning between -0.40 and 1.40 V (vs. SCE) at 20 mV/s (The first and last 40 seconds of data have not been shown due to lack of any ECL signal) 26. In this case the ECL emission is induced by the oxidation of both coreactant and nano hybrid using a well-known mechanism mentioned earlier, Bard’s and coworkers’ 39. The PbS-OA map in Figure 1a shows one peak in NIR region at 975 nm and 1.18 V potential in forward scan and its onset is 0.56 V. Two additional peaks in red (680 nm, 1.40 V) and NIR (984 nm, 1.02 V) regions are detected for PbS-BDY hybrid which result in a total of three distinct peaks (Figure 1b). The 1.18 V NIR peak of forward scan in Figure 1a shows a shift to 0.94 V in presence of BDY (Figure 1b). In comparison with the PL spectra of PbS-


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OA, PbS-BDY 24 and ECL spectra of sole BDY 40 in Figure 1, it could be concluded that the ECL emission at 680 nm and 984 nm are from BDY and PbS NCs, respectively. The mechanism of the generated ECL signal for PbS-BDY nanohybrid system in solution and at surface of electrode have been described previously 26. In order to investigate the complicated mechanism of the ECL signal generation, we apply multivariate analysis methods to the ECL data.

Figure 1. ECL map of a) PbS-OA and b) PbS-BDY in CH2Cl2 containing 0.1 M TBAPF6 during a potential scan cycle between -0.40 and 1.40 V vs. SCE at scan rate of 20 mV/s. The first and the last 40 seconds of data have not been shown here due to lack of any ECL signal. The 91th second is attributed to potential switch point, 1.40 V. Multivariate Analysis The high background signal in spooling ECL spectra was subtracted. Then, the MCR-ALS was used for resolving the ECL spooling spectra to its profiles in potential (Time) and wavelength modes. To the researchers’ knowledge, this is the first report in application of multivariate analysis on three dimensional (3D) ECL data. Likewise, the resolving of the fluorescence which leads to estimation of pure excitation and emission spectrums, resolution on the 3D ECL data would result in pure ECL and emission profiles. The ECL profiles show the relative concentration of 7 ACS Paragon Plus Environment


electroactive emitter species produced at the electrode surface region for each potential (like i/E curves in cyclic voltammetry assays). PbS-OA NCs Data Analysis In case of PbS-OA, the best data fits the MCR-ALS model is achieved by considering two components as shown in Figure 2(a,b). The obtained ECL profiles in potential mode have identical maximum, while the red line has earlier onset which shows the oxidation in lower potentials. Moreover, the corresponding emission spectra in wavelength direction presents about 100 nm difference in maxima. As mentioned earlier, there are two different energy states in NCs, which the core state has broader band gap in respect to the surface state. The outcome of multivariate analysis for PbS-OA ECL data is in accordance with the theory of NCs energy states. The surface state has a peak with lower energy (red line in Figure 2b) and has an oxidization earlier than core state (blue line in Figure 2a) at electrode surface. It should be noted that OA is not an electro and/or luminescence active specie in PbS-OA system; therefore, its conjugation on PbS NCs does not affect the surface or core states properties. 0.3

600

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Core

0.25

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Core

Surface

ECL Inensity

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.2 0.15 0.1

Surface

400

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0 0.4

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0.9

E / V vs. SCE

1.15

1.4

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0.3

500

c ECL Intensity

Surface BDY Core

0.25

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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d

300 200 100

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0

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E / V vs. SCE

500

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Wavelength / nm

Figure 2. The calculated profiles from applying MCR-ALS model on PbS-OA (a,b) and PbSBDY (c,d) data. ECL (a,c) and Emission (b,d) profiles. PbS-BDY NCs Data Analysis According to (Figure 1), the exchange of OA with BDY as surface ligand for PbS NCs has brought some new features in the ECL map of PbS-BDY NCs. BDY ECL signals around 680 nm which almost has a symmetric pattern in course of potentials (Figure 1b). However, Hesari et. al.

40

previously has reported ECL of BDY in solution but BDY shows only two significant peaks in forward scan without any signals in backward scan or symmetric pattern. In NIR region (900-1000 nm) related to the ECL of PbS NCs, the illustrated pattern is more complicated in respect to the PbS-OA (Figure 1a). In addition to the forward peak in NIR, there is another ECL peak at reverse scan at 1.02 V (Figure 1b). According to the results of previous part, it is revealed that core and surface states of PbS NCs together are responsible for signals in NIR region. So, there is a necessity to resolve pure profiles of involved species in PbS-BDY NCs in order to discover the mechanism of ECL signals in this system. It is expected, due to the appearance of ECL signal in 680 nm, the best MCR-ALS model with meaningful profiles (physical and mathematical aspects) will be achieved by three factors.



Drawing on previous published reports 24, 26, 40 and consequence gained by application of MCRALS on PbS-OA data (Figure 2(a,b)), it could be concluded that black, blue and red lines in Figure 2d are the emission spectrums of BDY, core and surface states of PbS QDs. As anticipated, the ECL profile of BDY (black line in Figure 2c) shows a symmetric pattern with a small peak at onset of forward scan. The peak potential of core state in PbS-BDY NCs shifts to lower oxidation potentials (1.12 V) with regard to the PbS-OA NCs. This pattern is almost identical to PbS-OA in course of potentials and also shows only one peak in forward scan. The estimated ECL profile for surface state in PbS-BDY NCs shows an unusual pattern in respect of the surface state in PbS-OA NCs. Meaning that, a peak located at 1.02 in reverse scan is higher than potential of forward scan (0.92 V). Analysis of Different Scan Rates Data In order to investigate the effect of potential scan rate on PbS-BDY NCs, the data acquisition is performed at 50, 100 and 200 mV/s (Figure S1). The spooling ECL maps display high consistency with 20 mV/s. Supposing all the emission spectra are identical in all scan rates, MCR-ALS could apply an augmented data in potential (time) direction in order to find their ECL profiles in higher scan rates. The results of simultaneous analysis of four scan rates using MCR-ALS are illustrated in Figure 3. The shape of ECL core states are approximately identical in all scan rates, although its maxima changes. This can be taken as a proof for the independent ECL signal of core states in PbS-BDY system. There is a systematic and significant relationship between surface state of PbS NCs and BDY. By increasing the scan rate, dramatic changes happen to the shapes of BDY and surface state ECL profiles. In all scan rates, the onset of BDY and surface states ECL profiles are the same. The peak in reverse scan for surface state has diminished as well as symmetric pattern of BDY.


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Intensity / a.u.

Intensity / a.u.

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Core

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BDY

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Core

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0 41

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Time / Sec 0.3 Surface BDY Core

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0.15 0.1 0.05 0

0 10

15

20

6

25

Time / Sec

9

12

15

Time / Sec

Figure 3. The calculated ECL profiles for augmented data of PbS-BDY in different scan rates, a) 20 mV/s b) 50 mV/s c) 100 mV/s d) 200 mV/s

Excited State Charge Transfer In PbS-OA NCs, the surface state has identical pattern in the core state even with different scan rates 26, which unveils the electrochemical inertness of OA ligands in PbS NCs. On the other hand, conjugation of BDY to PbS NCs changes ECL behavior of surface state in such a way that there are numerous common features in their ECL profiles (Figure 3). In comparison to its profile with BDY, it is revealing that its peaks at forward and reverse scan is completely dependent on BDY



pattern, which accompanies BDY profiles at all scan rates. The minimum between two peaks is located at the same potential when BDY reached to a maximum. The deconvolution of surface states at 20 mV/S (Figure S2a) exhibits that its minor peak at 0.74 V is located at the same potential of BDY shoulder in forward scan (Figure S2b). Consequently, it would not be a coincidence for all these evidences that correlate the ECL patterns of surface state of PbS NCs with BDY; hence, it can be assumed that there should be a strong relationship between these two. In 2011, Wang and co-workers stated that energy level of HOMO and LUMO of PbS NCs and BDY ligands could be considered as sufficient conditions for excited-state charge transfer from the BDY to the PbS NCs core

24.

But in the present research and through using ECL analysis

instead of PL (which mainly dealing with core state), it is revealed that the excited-state charge transfer could be from BDY to the surface state of PbS NCs. In line with our results the ECL efficiencies calculated by Hesari et. al. 26, 40 demonstrated the quenching of BDY and the emission from PbS NCs. The ECL efficiency of BDY at PbS-BDY hybrid system in respect to the BDY 40 decreased for different scan rates while the ECL efficiency of PbS NCs at PbS-BDY hybrid system increased in comparison to PbS-OA. The patterns of ECL for these two energy bands show that charge transferring did not happen for all potentials, especially for higher positive potentials when BDY reaches to maximum at the end of forward scan (t=91 s, E=1.40 V at 20 mV/s). BDY has a complete access to management of emission from the PbS NCs surface states through a state with higher energy with respect to the surface states of PbS NCs. This proposes that excited-state charge transfer process happens between excited form of BDY+ and surface state of PbS NCs. It is worth to note that excited-state charge transfer happens at all scan rates, meaning that this process is independent of scan rates.


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As reported by Hesari et. al.

40,

at higher positive potentials sole BDY undergoes the second

oxidation generating BDY dication (BDY2+) and producing two BDY•+ in reaction to neutral BDY in the solution. But this cannot happen in PbS-BDY NCs since the emission intensity for BDY at the end of forward scan is high. If this emission is produced by BDY•+, it should be quenched by excited-state charge transfer process and it should appear in NIR region by surface states of PbS NCs. Therefore, there could be another mechanism for BDY emission in higher positive potentials in PbS-BDY NCs. Adsorption on Platinum Electrode Surface The heterogeneous electron transfers, induced the ECL generation, are deeply dependent on the electrode surface and material 41, 42. Adsorption processes along the electrochemical reactions on electrode surface and it often leads to unwanted signals and to more complicated results 43. In both raw (Figure 1b) and resolved data (Figures 2c, S2a and 3) there are witnesses which could be related on adsorption behavior of BDY and surface state of PbS NCs which will be discussed further in this section. There are two main challenges: a) the adsorption process has been seen only for PbS-BDY NCs and not for PbS-OA NCs; b) the core states of PbS NCs did not show any adsorption behavior in any of scan rates. These challenges lead to intricate electrochemical behavior of PbS-BDY NCs. The BDY could not be the reason for adsorption in PbS-BDY NCs system since the BDY has no tendency to the electrode surface

40.

As a result, different surface conditions of PbS NCs in

encountering OA and BDY should be the main reason. Nanocrystals are consisted of hundreds and thousands of atoms with their surfaces encircled by small facets, vertex and edge sites. PbS NCs often adopt cuboctahedral shape, terminated by (111) and (100) facets with the hexagonal and square arrangements of surface atoms 13 ACS Paragon Plus Environment

29, 30.

Recent


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computational studies reveal that (100) facet of PbS NC presents a chequerboard arrangement of lead and sulfur atoms. On the other hand, (111) facet of PbS NCs presents a hexagonal layer of Pb atoms 29. OA (C18H34O2) is the most common and applicable ligand used for PbS NCs synthetization and surface capping. The experimental and computational efforts prove that at (111) facet there is a strong and high-ordered binding between Pb atoms and anion form of OA (oleate ion). But at (100) facet OA form bidentate bridges between Pb and S atoms through –COOH groups

28, 29.

Also, a

high-density packing, with one OA (-COOH) per PbS-pair, is unfavorable at (100) facet because of steric repulsions between the oleyl tails. The main common feature of BDY and OA is the carboxylic acid group but the acidity strength of BDY with its electron-withdrawing group is in a higher degree in respect of OA with alkyl electron-donating group. Also, the structural comparison between OA and BDY reveals that BDY has a larger molecule size. Hence, the BDY has more tendency to binge to the (111) facet rather than (100) facet. Moreover, due to the steric hindrance, the overall capping density of (111) facet and especially (100) facet of PbS NCs in the case of BDY is much less than OA. By considering all these phenomena, it is acceptable that there are more unpassivated S atoms at (100) facet of PbS NCs. Consequently, these unpassivated S atoms would become a force to the adsorption of PbS-BDY NCs to the surface of Pt electrode. In case of time scale in this experiment (ECL assay of PbS-BDY NCs), the driving force for adsorption of sulfur groups will be physical and not electrochemical 44, 45. Although thiol molecules adsorb on a platinum surface, the chemisorption was reported too slow 46, 47. The ECL reactions in PbS-BDY system is defined in coreactant pathway, where the oxidation kinetic of TPrA

13, 41, 42

would

strongly affect the ECL patterns of PbS-BDY in different scan rates. As the unpassivated S atoms


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adsorb spontaneously, adsorption will occur in all scan rates and it is not dependent to the scan rates. Core state not showing an adsorption pattern during the ECL assay in PbS-BDY NCs which could be considered as a problem. There are some reports in relation to NC thin films on electrode surface which show that the electrochemical and even ECL of NCs are not dependent on adsorption 48.

Hence, the ECL pattern of core state is in agreement with the nature of NCs. Moreover, the

ECL pattern of surface state which should be like core state, shows an unusual pattern due to the strong relation and dependency on BDY. Nevertheless, it would be true that there is some adsorbed NCs in PbS-OA which demands more researches and assays. Conclusion This work demonstrates that accompaniment of effective experimental data acquisition with multivariate data analysis methods lead to the systematic knowledge about ECL’s behavior of electrogenerated species and transfer mechanism in PbS-BDY NCs hybrid. Multivariate analysis of ECL spooling spectra by MCR-ALS method resolves the unique emission and ECL profiles of surface and core states. Accessibility of LUMO of PbS NCs surface states for the LUMO of excited form of BDY+ leads to the electronic interchange between BDY and PbS NCs in all scan rates. The presence of symmetric patterns in ECL map and calculated profiles of PbS-BDY NCs reveal the adsorption process due to the essence tendency of unpassivated sulfur atoms on the surface of PbS NCs. The reported results contribute to a better understanding of the mechanism and the operating condition for the ECL from NCs. They also pave the way for the multivariate application in ECL analysis.



Supporting Information Brief explanation for MCR-ALS, Spooling spectra maps for PbS-BDY NCs in four different potential scan rates, Deconvolution of ECL profiles of surface state of PbS NCs and BDY in PbSBDY hybrid. Acknowledgments This research was supported by Institute for Advanced Studies in Basic Sciences (IASBS). We are also grateful to Prof. Zhifeng Ding and Dr. Mahdi Hesari for their generosity with providing raw data and their advises on the manuscript.


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Unveiling Adsorption of Boron Dipyrromethene Conjugated PbS

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