Porphyrinic MOF Film for Multifaceted Electrochemical Sensing

Article abstract of DOI:10.1002/anie.202107860

Electrochemical sensors are indispensable in clinical diagnosis, biochemical detection and environmental monitoring, thanks to their ability to detect analytes in real-time with direct electronic readout. However, electrochemical sensors are challenged by sensitivity—the need to detect low concentrations, and selectivity—to detect specific analytes in multicomponent systems. Herein, a porphyrinic metal-organic framework (PP-MOF), Mn-PCN-222 is deposited on a conductive indium tin oxide (ITO) surface. It affords Mn-PCN-222/ITO, a versatile voltammetric sensor able to detect redox-active analytes such as inorganic ions, organic hazardous substances and pollutants, including nitroaromatics, phenolic and quinone-hydroquinone toxins, heavy metal ions, biological species, as well as azo dyes. As a working electrode, the high surface area of Mn-PCN-222/ITO enables high currents, and therefore leverages highly sensitive analysis. The metalloporphyrin centre facilitates analyte-specific redox catalysis to simultaneously detect more than one analyte in binary and ternary systems allowing for detection of a wide array of trace pollutants under real-world conditions, most with high sensitivity.

Full text of DOI:10.1002/anie.202107860

Angewandte
Research Articles
Chemie
How to cite:
International Edition: doi.org/10.1002/anie.202107860
German Edition: doi.org/10.1002/ange.202107860
Electrochemical Sensors
Hot Paper
Porphyrinic MOF Film for Multifaceted Electrochemical Sensing
Zhenyu Zhou, Soumya Mukherjee, Shujin Hou, Weijin Li,* Martin Elsner, and
Roland A. Fischer*
Abstract: Electrochemical sensors are indispensable in clinical
diagnosis, biochemical detection and environmental monitor-
ing, thanks to their ability to detect analytes in real-time with
direct electronic readout. However, electrochemical sensors are
challenged by sensitivity—the need to detect low concentra-
tions, and selectivity—to detect specific analytes in multi-
component systems. Herein, a porphyrinic metal-organic
framework (PP-MOF), Mn-PCN-222 is deposited on a con-
ductive indium tin oxide (ITO) surface. It affords Mn-PCN-
222/ITO, a versatile voltammetric sensor able to detect redox-
active analytes such as inorganic ions, organic hazardous
substances and pollutants, including nitroaromatics, phenolic
and quinone-hydroquinone toxins, heavy metal ions, biological
species, as well as azo dyes. As a working electrode, the high
surface area of Mn-PCN-222/ITO enables high currents, and
therefore leverages highly sensitive analysis. The metallopor-
phyrin centre facilitates analyte-specific redox catalysis to
simultaneously detect more than one analyte in binary and
ternary systems allowing for detection of a wide array of trace
pollutants under real-world conditions, most with high sensi-
tivity.
analyte-specific redox potentials that open specific windows
for detection, electrochemical sensors have spurred consid-
erable research interest and have been widely used in clinical
diagnosis, biochemistry and environmental monitoring.[1] Be-
sides potentiometric sensors, voltammetric (= Volt-/ Ampero-
metric) sensors are the most common kind. Here analyte-
specific redox reactions are detected by measuring the
corresponding electrical current in response to changes in
the applied electrochemical potential.[1b,2] This has propelled
cyclic voltammetry (CV) as a front-runner for sensing redox-
active analytes. Here, the applied potential at the working
electrode is varied back and forth at a certain scan rate over
a specific potential range. By plotting the current response
versus the applied potential, a cyclic voltammogram trace is
obtained,[3] which is symmetric in the case of reversible redox
reactions at the electrode surface and non-symmetric other-
wise. Despite such successful technological examples as the
Clark electrode for detection of oxygen,[4] the high sensitivity
and universality of electrochemical sensors (i.e., the ability to
detect a wide range of analytes such as organic pollutants,
heavy ions, biological markers and azo dyes) remain unmet
challenges until today.
Introduction
Metal-organic frameworks (MOFs), composed of organic
linkers and inorganic metal nodes (or clusters), have drawn
enormous attention due to their well-defined porous struc-
tures with high surface areas, good stability and tailored
functionalities.[5] These features lead to an increased propen-
sity for the analytes to get concentrated and to expedite rapid
mass transfer of the analytes across a large surface area. This
in turn amplifies the voltammetric current as a characteristic
signal response and thereby enhances sensitivity, making
MOFs promising for electrochemical sensing.[1a,6] On the
other hand, thanks to their p-conjugated macrocyclic struc-
tures, porphyrins and metalloporphyrins demonstrate good
electron transfer and selective redox catalytic properties, thus
befitting to electrochemically sense several target analytes.[7]
Considering these advantages of MOFs and porphyrins,
we develop porous porphyrinic MOFs (PP-MOFs) treating
these as molecular building blocks. In the pursuit of electro-
chemical sensing, PCN-222 (PCN, porous coordination net-
work), a prototypical PP-MOF constructed from Zr6 cluster
and tetrakis(4-carboxyphenyl)-porphyrin (TCPP) ligand is
Electrochemical sensing stems from changes in the
detectable electric signal caused by the interactions between
electrode and specific analytes. Thanks to its advantages, such
as quick response, high efficiency, simple operation, and
[*] Z. Y. Zhou, Dr. S. Mukherjee, Dr. W. J. Li, Prof. R. A. Fischer
Chair of Inorganic and Metal-Organic Chemistry, Department of
Chemistry, Technische Universitꢀt Mꢁnchen
Lichtenbergstraße 4, 85748 Garching b. Mꢁnchen (Germany)
E-mail: wj.li@tum.de
roland.fischer@tum.de
S. J. Hou
Physics of Energy Conversion and Storage, Physic-Department,
Technische Universitꢀt Mꢁnchen
James-Franck-Str. 1, 85748 Garching b. Mꢁnchen (Germany)
Prof. M. Elsner
Chair of Analytical Chemistry and Water Chemistry, Technische
Universitꢀt Mꢁnchen, Department of Chemistry
Lichtenbergstraße 4, 85748 Garching b. Mꢁnchen (Germany)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202107860.
used. PCN-222
features ultrahigh surface areas
(> 2000 m2 gÀ1).[8] Its 1D hexagonal mesopores ( ꢀ 3.0 nm)
replete with porphyrin sites (Figure 1a) endow it as a potent
lead to enable electrocatalytic sensing.[9] In pursuit of electro-
chemical applications, integration of PP-MOFs onto solid
surfaces in the form of films is a prerequisite that we achieve
herein.[10]
ꢂ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution Non-Commercial
NoDerivs License, which permits use and distribution in any
medium, provided the original work is properly cited, the use is non-
commercial and no modifications or adaptations are made.
&&&&
ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Figure 1. (a) Structures of the building blocks: Mn-TCPP ligand and Zr6 cluster sustaining Mn-PCN-222; the framework structure of Mn-PCN-222
with 1D triangular microchannels and hexagonal mesochannels. H atoms are omitted for clarity. Color Scheme: C, yellow; O, red; N, blue; Cl,
olive green and Mn, cyan. (b) Schematic illustration of the modular assembly driven synthetic route leading to a Mn-PCN-222/ITO electrode.
(c) Simulated and experimental PXRD patterns of the bulk Mn-PCN-222 and GIXRD patterns of the Mn-PCN-222 film fabricated by modular
assembly. (d) SEM images of the Mn-PCN-222 film casted on ITO glass. (e) ATR-IR spectra of free H2TCPP ligand, Mn-TCPP ligand and Mn-PCN-
222 film.
Recently, Mn-PCN-222 modified with poly-glutamic acid
has been utilized earlier as an electrochemical H2O2 sensor.[9b]
Nevertheless, the suitability of Mn-PCN-222 rests upon
rationally combining metalloporphyrin redox sites with
To the best of our knowledge, this is the first report of an
electrochemical sensor built from modular assembly.
porosity, each enabling facile electron transfer and analyte Results and Discussion
transport. This, in unison, leads to a miscellany of sensing
performances while detecting several analytes. Low detection
limits are manifested herein, that cover both inorganic
substrates and organic pollutants. Also, critical analysis of
such a multifaceted detection process remains an uncharted
territory. Herein, we forge ahead with our detailed electro-
chemical studies to fill this knowledge gap by selecting to
study Mn-PCN-222 as a versatile sensor.
In this study, a facile and straightforward modular
assembly technique for liquid-phase depositing of Mn-PCN-
222 (manganese metalloporphyrin) film on conductive in-
dium tin oxide (ITO) glass leads us to fabricate the Mn-PCN-
222/ITO electrode (Figure 1b).[8b,11] Herein, solvothermally
synthesized PP-MOF nanoparticles (NPs) serve as thin film
precursors. Thanks to hydrophobicity, these PP-MOF NPs
spontaneously spread out as a thin film on water, from which
they are easily transferred to the substrate. This simple
stamping results in a very homogeneous thin film of
assembled NPs. Overall, our observations introduce modular
assembly as a reliable and efficient way of preparing PP-MOF
film electrodes for multifunctional electrochemical sensors.
Powder X-ray diffraction (PXRD) patterns of the Mn-
PCN-222 powder and its film obtained via modular assembly
are shown in Figure 1c. These diffraction patterns are found
to be in good agreement with the simulated XRD profile,
suggesting bulk phase purity of the crystallized samples and
confirming that the obtained thin film solely comprise the
Mn-PCN-222 crystallites.[8,12] Scanning electron microscopy
(SEM) of an Mn-PCN-222 film-modified ITO electrode
shows the film to be composed of rod-shaped Mn-PCN-222
crystallites (length ꢀ 1 mm) evenly dispersed on the ITO
surface (Figure 1d). In principle, if the properties of building
blocks are synergized, such a uniform coverage of Mn-PCN-
222 crystallites on the conductive surface of ITO should
indeed lead to an efficient electrochemical sensor.[10b] Energy-
dispersive X-ray spectroscopy (EDX) confirms a uniform
distribution of Mn elements across the Mn-PCN-222 thin film
(Figure S1). Attenuated total reflection infrared spectroscopy
(ATR-IR) spectra and ultraviolet-visible spectroscopy (UV/
Vis) spectra are recorded to further identify the compositions
of the prepared Mn-PCN-222 film (Figures 1e and S2). In
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
contrast to the free H2TCPP ligand, the metallisation of Mn-
TCPP ligand is evidenced by disappearance of the N-H
stretching vibration at 960 cmÀ1 and appearance of a new
[8b,13]
peak at 1008 cmÀ1 assigned to Mn N bonds.
The latter
À
signature is also found in the Mn-PCN-222 film (Figure 1e).
This is well aligned with the EDX and UV/Vis spectra
(Figures S1 and S2). Moreover, compared to the spectrum of
Mn-TCPP, Mn-PCN-222 films exhibit an absence of charac-
À1
teristic peaks around 1700 cmÀ1 (C O bonds) and 1270 cm
À
=
À1
(C O bonds) whereas strong peaks at 1420 cm (COO
symmetric stretch bonds) appear, reflective of the carboxyl
group coordinating to the Zr6 centres in Mn-TCPP.[8b,14] To
verify the loading of Mn-PCN-222 onto the ITO electrode
each time, UV-vis spectra of 10 independent samples are
investigated and the absorbances at 465 nm are noted
(Table S1).[13a] An average absorbance of 0.1421 a.u. (Fig-
ure S3) is observed, whereas its excellent reproducibility is
evidenced by the low relative standard deviation (RSD) of
just 3.0%. The porosity of bulk Mn-PCN-222 is examined by
N2 adsorption experiments and the typical type-IV isotherm is
accompanied by a steep rise at ca. P/P0 = 0.3, suggesting both
micro and mesoporosity (Figure S4). The pore size distribu-
tion profile indicates two types of pores with sizes of ꢀ 1.3 nm
and ꢀ 3.0 nm, assigned to the coexistence of 1D triangular
microchannels and hexagonal mesochannels, respectively in
the framework structure (Figure 1a).[8a] Applying Rouquerol
criteria to the 77 K N2 isotherm, the Brunauer-Emmett-Teller
Figure 2. (a) CV profiles of the Mn-PCN-222/ITO electrode in 0.5 M
NaCl solution in the presence or absence of 100 mM NB at a scan rate
of 50 mVsÀ1. Inset: an enlarged CV curve obtained in the absence of
100 mM NB for comparative clarity. (b) CV curves recorded on different
electrodes targeting the reduction of 100 mM NB. (c) Schematic
illustration of the plausible mechanism of NB reduction on the Mn-
PCN-222/ITO electrode.
À0.77 V in the presence of 100 mM NB. This could be
attributed to the direct reduction of NB to phenylhydroxyl-
amine in neutral medium, concomitant with the transfer of
four electrons and protons (Figure 2c).[16b,c] Relying upon this
reduction, the modified electrode shows potential as an
electrochemical sensor for NB detection. The detailed process
and putative mechanisms for the electrocatalytic NB reduc-
tion on the Mn-PCN-222/ITO can be described as follows:
first, the large surface area of Mn-PCN-222 and the highly p-
conjugated porphyrin ring (p-p interactions with NB) con-
tribute to the aggregation of NB molecules; on top of this, the
porphyrin ligands feature intrinsic redox activity, which would
contribute to the electrochemical sensing of redox-active NB
analytes; under an external voltage, the MnIII porphyrin
centres of Mn-PCN-222 are reduced to MnII porphyrins state
and serve as electron donors. Meanwhile, thanks to the strong
electron-withdrawing nature of the nitro group, NB acts as an
excellent electron acceptor, thus affording an electron donor-
acceptor (EDA) system formed between the porphyrin
centres and the NB molecules; this EDA system facilitates
sequential electron transfer to the nitro group, followed by
protonation, thereby culminating in the reduction of
NB.[7a,16c,18]
To further evaluate NB reduction performances of the
Mn-PCN-222/ITO electrode and to rationally compare,
a) bare ITO; b) non-metallated PCN-222/ITO; and c) Mn-
TCPP/ITO electrodes are also prepared separately. As shown
in Figure 2b, the bare ITO shows a weak current response
with À0.06 V negative shifts for NB compared to that of Mn-
PCN-222/ITO electrode, clearly revealing its poor response as
a redox catalyst towards NB. Regarding the as-prepared
modified electrodes, the Mn-PCN-222/ITO electrode also
exhibits superior electrochemical behaviour. A three-fold
increase in reduction current is observed against the non-
(BET) surface area is evaluated as high as 2013 m2 gÀ1.[15]
A
methanol sorption isotherm recorded on the environmentally
controlled quartz crystal microbalance (BEL-QCM) confirms
the high porosity of film-desposited NPs (Figure S5). The
large 1D mesochannels and high surface area, in principle,
should prove beneficial to enable rapid analyte aggregation
and mass transport.[8a,9a,b]
After characterizing the prepared Mn-PCN-222 film, as
a proof of concept, we begin to critically examine their
functions as electrochemical sensors. Before testing any other
hazardous inorganic and organic substances, one of the most
common representatives of nitroaromatic compounds, nitro-
benzene (NB), is chosen as a model toxin. Chasing after CV
based sensing, this compound allows us to optimize the
critical parameters, e.g., scan rate, pH, accumulation time. It
also offers to study selectivity, stability, recyclability and
reproducibility, in which the currents of CV reduction peaks
correspond to signal readouts.
Both toxicity and carcinogenicity aspects are well-docu-
mented for nitroaromatics, particularly nitrobenzene (NB)
and their derivatives. These can pose threats to human health
and the environment even at low concentrations of ca.
2 mgLÀ1 [16]
Rapid and efficient detection of NB in water is,
.
therefore, a matter of high societal and environmental
relevance. To begin with, as an electrochemical sensor for
NB detection, the Mn-PCN-222/ITO electrode is studied and
the corresponding sensitivity and selectivity of detection are
determined. In the absence of NB, a pair of redox peaks are
observed on the CV curve at ca. À0.45 V and À0.32 V
(Figure 2a), corresponding to the reversible couple of MnIII/
MnII on Mn-PCN-222.[17] The Mn-PCN-222/ITO-modified
electrode exhibited a sharp, irreversible reduction peak at
&&&&
www.angewandte.org
ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
These are not the final page numbers!

Angewandte
Research Articles
Chemie
metallated PCN-222/ITO electrode. The better electrocata-
the current peak values remain nearly constant after an
lytic activity towards NB is attributed to the faster charge
hopping and improved redox activity after coordination of
MnIII to the porphyrin centres (Figure 2c).[9d,10c,19] Moreover,
among the metallated M-PCN-222 (M = Mn, Fe, Co, Cu, and
Zn) variants, Mn-PCN-222/ITO ranks as the front runner for
NB reduction (Figures S6–S8). Meanwhile, a Mn-TCPP/ITO
electrode is also prepared for comparison. Quantified by UV/
Vis, the catalytically active Mn-TCPP are equally loaded on
both Mn-TCPP/ITO and Mn-PCN-222/ITO electrodes (Fig-
ure S9). The superiority of Mn-PCN-222 with high surface
area, large 1D mesochannels and ordered accessible active
sites is evidenced by the comparative examination of Mn-
TCPP/ITO electrodes (Figure 2b).
accumulation time > 90 s, presumably due to NB saturation of
the modified electrode under a dynamic equilibrium. Inter-
estingly, a rapid saturation is achieved at a high NB concen-
tration (100 mM). This could be ascribed to the high surface
area and large 1D mesochannels of Mn-PCN-222, leading to
rapid analyte aggregation and mass transport.[8a,9b] According
to these results in Figure S13, 90 s is selected as the optimum
accumulation time for further investigation.
The feasibility of Mn-PCN-222/ITO electrode for NB
detection is confirmed under optimized conditions with the
addition of different concentrations of NB (Figure 3a; for
zoomed views of the low concentrations, see Figure S14).
Notably, the corresponding reduction peak currents (I)
increased linearly with the increase in NB concentration (c)
ranging from 0.08 up to 700 mM with high correlation
coefficients (R2 > 0.999). According to the criterion of United
States of Environmental Protection Agency (USEPA), the
acceptable limit of NB in water is 2 mgLÀ1 (16.25 mM),[16a,c]
falling within the detectable range of the modified electrode,
thus suggesting its potential for NB detection. In addition, the
sensitivity is determined to be 0.6479 mAmMÀ1 cmÀ2 (0.08–
100 mM) and 0.4468 mAmMÀ1 cmÀ2 (100–700 mM), respective-
ly. The slight difference of sensitivity is likely to be an
outcome of the following concentration-guided factors: a) the
abundance of catalytic sites implies their easy accessibility for
the absorbed NB molecules at low concentrations; b) never-
theless at higher concentrations, these sites become seques-
tered owing to saturation with NB, thus compromising the
sensitivity.[9a–c] The limit of detection (LOD) is calculated as
low as 0.03 mM (3 ꢁ Sb/slope, Figure S15, Table S2). Compared
to the previously reported modified electrodes for NB
detection, the proposed Mn-PCN-222/ITO electrode sets
a benchmark analytical response towards NB in terms of
a wide linear response and low LOD (Table S3). Its electro-
catalytic activity towards NB detection stands out, and can be
correlated to its large surface area, expedited preconcentra-
tion, rapid mass transport and high density of the accessible,
ordered active sites in Mn-PCN-222.[8a,9a,b]
We evaluated the selectivity of the Mn-PCN-222/ITO
electrode towards NB (50 mM) in the presence of 10 fold
molar excess of other typical interferents (Figure S16). It can
be seen that common metal ions and biological species only
induce negligible interference (< 2%) whereas phenolic
compounds, quinone and benzene derivatives only trigger
measly effects (< 4%) on the NB detection performance.
These results indicate the potential of the modified electrodes
for selectively detecting NB, even in the presence of an excess
of these potentially interfering species.
To further confirm practical viability, the modified
electrode is employed to detect NB in real water samples
(tap water and river water) with addition of NB of different
concentrations (10 mM, 20 mM and 30 mM) and analyzed by
cyclic voltammograms (Table S4). The Mn-PCN-222/ITO
electrode featured excellent recoveries (the percentages of
measured concentrations to the corresponding added con-
centrations) towards NB detection (96.2 to 102.5%) in real
samples, suggestive of its suitability as NB sensor to detect
water contamination.[16b,c]
The effect of scan rate (ranging from 30 to 150 mVsÀ1
)
towards NB reduction is assessed by cyclic voltammetry
(Figure S10). The reduction peak current increased linearly
with respect to the square root of scan rate with a correlation
coefficient R2 of 0.9996. This suggests a typical diffusion-
controlled process to drive the electrochemical reduction of
NB at the modified electrode.[16b–d] Moreover, the electron
transfer number for cathodic NB reduction is estimated as 3.8
according to Lavironꢀs equation,[20] suggesting a 4 electrons
transfer process. Conversely, the chronoamperometric re-
sponse of Mn-PCN-222/ITO towards different NB concen-
trations reveals the diffusion coefficient for NB to be 6.178 ꢁ
10À6 cm2 sÀ1 based on Cottrellꢀs equation (Figure S11).[9a,c,d]
pH is another key parameter that may profoundly
influence the performance of a voltammetric sensor, i.e., the
respective redox potential and the kinetics of the redox
reaction. Herein, CV curves of Mn-PCN-222/ITO electrode
are recorded across different pH (5 to 9) (Figure S12). CV
profiles recorded at a relatively high H+ concentration (pH 5)
exhibit an increased reduction current at À1.0 V, correspond-
ing to a side reaction of hydrogen evolution reaction (HER).
Concurrently, during reduction, a low H+ concentration (at
pH 9) hinders the subsequent NB reduction due to the
increased difficulty in protonation. In comparison with
solutions of varying pH, the maximum peak-current response
towards NB reduction is found to be achieved at pH 7.[16b–d]
Hence, pH 7 is concluded to be the optimum pH for detecting
NB on the Mn-PCN-222/ITO electrode. Moreover, with
increasing pH, the reduction peak potential (Ep) exhibits
a linear shift to the negative end with the corresponding
equation of EpV = À0.0581 pH-0.368. The slope of
58.1 mVpHÀ1 is found in close agreement with the theoretical
value, 59 mVpHÀ1, indicating that an equal number of
protons and electrons are transferred during NB reduction.[21]
Simply put, the pH dependence of electrochemical behaviour
reflects the influence of H+ on NB reduction while studying
the electrode, which in turn corresponds to the mechanism of
NB reduction.[16b–d]
The influence of another parameter, viz. accumulation
time, is also investigated by altering the accumulation time
just before measuring the cyclic voltammograms (Figure S13).
Under a low NB concentration of 10 mM, the CV response
currents increase as a function of the time of accumulation.
This is attributed to the increased amount of NB molecules
aggregated on the Mn-PCN-222/ITO electrode. Thereafter,
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Figure 3. CV curves of the Mn-PCN-222/ITO electrode towards the detection of (a) NB, (b) 4-NBZ, (c) 4-NP, (d) NB and 4-NBZ, (e) 4-NP and 4-
NBZ, (f) 1,3-DNB at different concentrations; inset shows linear trend of the peak current versus concentration of corresponding analytes.
Material stability, recyclability and reproducibility are
important parameters for electrochemical sensors. Mn-PCN-
222/ITO exhibits excellent stability substantiated by the
retention of nearly 97.3% of its initial response after 30 days
(Figure S17). The high stability originates from the stable Zr6
cluster and strong chelating effect between MnIII and the
porphyrins.[8a,9b,c] The electrode also shows good recyclability
with a RSD of 4.98% (< 5%) for 40 measurement cycles
(Figure S18). Meanwhile, a comparison of the XRD patterns,
SEM images, UV/Vis spectra and Raman spectra on the
modified electrodes reveals identical crystalline features and
morphologies, changeless coating condition as well as same
molecular structures before and after consecutive detection
measurements, indicating their excellent electrochemical and
mechanical stability (Figures S19–S23). Furthermore, 10 in-
dependent Mn-PCN-222/ITO electrodes are examined. A
RSD of 2.67% (< 5%) (Table S5) suggests outstanding
reproducibility of the modified electrode.
Aligned with the observations made in our foregoing
experiments, the feasibility of Mn-PCN-222/ITO for NB
detection becomes clear. To verify the versatility of the
proposed modified electrode, electrochemical detection of
other nitroaromatic compounds (NACs), viz. 4-nitrobenzal-
dehyde (4-NBZ), 4-nitrophenol (4-NP), 1,3-dinitrobenzene
(1,3-DNB) are investigated in individual and/or simultaneous
modes (Figures 3b–f). Although a reduction peak appears
due to the presence of a nitro group, the peak potentials for 4-
NBZ and 4-NP register clear shifts compared to that of NB
due to the withdrawal or donation of electron density at the
nitro group by different substituent groups at the NAC
rings.[16d,22] The -CHO group in 4-NBZ withdraws the electron
density from the aromatic ring, then resulting in an electron-
deficient nitro group that is more easily reduced. Therefore,
a positive shift of reduction potential is observed for 4-NBZ
(À0.60 V). On the contrary, the electron-donating group -OH
in 4-NP facilitates a negative shift of the reduction potential
to À0.95 V. As shown in Figures 3b,c, the Mn-PCN-222/ITO
exhibits linear responses to 4-NBZ (0.07–100 mM) and 4-NP
(0.5–100 mM) with high correlation coefficients (0.9991 and
0.9979, respectively). The LOD for the detection of 4-NBZ
and 4-NP detection are 0.030 mM and 0.106 mM, respectively.
Notably, the Mn-PCN-222/ITO shows a good analytical
response compared to 4-NBZ and 4-NP sensors reported in
the literature (Tables S6, S7). Figure 3d establishes the
simultaneous detection of NB and 4-NBZ with two well-
resolved and distinct reduction peaks. On top of this, Mn-
PCN-222/ITO detects NB and 4-NBZ mixtures in real water
samples (tap and river water) with addition of different
concentrations of NB (10 mM, 20 mM and 30 mM) and 4-NBZ
(10 mM, 15 mM and 20 mM) at appreciable recoveries of 93.6
to 103.9% (Table S8). Meanwhile, the Mn-PCN-222/ITO
shows highly linear responses to NB and 4-NBZ in the range
of 0.5 to 100 mM with LODs of 0.027 mM and 0.029 mM,
respectively. Similarly, the Mn-PCN-222/ITO also achieves
simultaneous detection (1–100 mM) of 4-NP and 4-NBZ with
LODs of 0.103 mM and 0.034 mM, respectively (Figure 3e).
Moreover, a clear electrocatalytic response despite the co-
existence of NB, 4-NBZ, 4-NP, suggests its usefulness in
simultaneously detecting a set of target NACs (Figure S24).
The proposed electrode could also recognize the number of
nitro groups in an individual NAC analyte, confirmed by the
two reduction peaks while detecting 1,3-DNB (Figure 3 f).[23]
&&&&
www.angewandte.org
ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
These are not the final page numbers!

Angewandte
Research Articles
Chemie
For 1,3-DNB sensing, the excellent linearity observed across
performance in sensing various analytes, viz., nitroaromatics,
0.08 to 100 mM with a LOD of 0.018 mM (based on peak b)
establishes comparable sensing performances versus the
hitherto reported electrodes (Table S9). Systematic studies
on NACs detection demonstrate high efficiency of the Mn-
PCN-222/ITO not only towards single-component systems,
but also in binary and ternary systems (Figures 3 and S24).
To further reveal the sensory versatility of Mn-PCN-222/
ITO, a series of voltammetric detections are performed on
other analytes across different classes, viz. phenolics and
quinone-hydroquinone, heavy metal ions, biological species,
as well as the azo dyes. The sensing performances are detailed
in Table 1 and Figures S25–S33. Upon comparing these
against reported sensors, excellence of Mn-PCN-222/ITO
becomes evident (Tables S10–16). Aligned with the demon-
strated analyte-specific responses, Mn-PCN-222/ITO can also
simultaneously detect mixtures of three analytes from three
distinct classes: CT, 1,4-BZQ and AQDS (Figure S34); Cu2+,
phenolics, quinone-hydroquinone, heavy metal ions, biolog-
ical species, as well as azo dyes. Sensitive detections with wide
linear ranges and low LODs are obtained with Mn-PCN-222/
ITO. The electrode shows high efficiency not only towards
single substrates, but also retains its high performance in
binary and ternary systems of increased complexity. An
extension to more complex systems can be foreseen, upon
conducting process/device engineering with this system. An
optimal combination of selectivity, long-term stability, repro-
ducibility and practicality of Mn-PCN-222/ITO is discovered.
In essence, Mn-PCN-222/ITO not only turns out amenable to
simple and efficient preparation methods, but also delivers
excellent voltammetric detection performance and, therefore,
holds great promise to design versatile electrochemical
sensors in the future. Building upon the compositional
modularity of M-PP-MOFs, varying intrinsic properties such
as, pore environments, redox-potentials/windows for each
(e.g., by choosing M properly or combining different M-PP-
MOFs in one sensor device) paves the way for a number of
related approaches, e.g., redox-potentials/windows are tune-
able by an ꢀ la carte catalogue of metal nodes and this is likely
to offer another dimension to the control of electrochemical
properties of M-PP-MOF based voltammetric sensors.
2À
Cr2O7 and Cd2+ (Figure S35); AA and UA (Figure S36),
suggesting itꢀs high upside potentials to fabricate a multi-
faceted sensory device in binary and ternary systems.
Conclusion
In summary, a fast and facile modular assembly method is
harnessed to cast porphyrinic Mn-PCN-222 film on the ITO
glass. The resulting Mn-PCN-222/ITO demonstrates versatile
voltammetric sensing of several analytes. The ordered incor-
poration of metalloporphyrin units as catalytic centers across
a periodic and porous Mn-PCN-222 architecture facilitates
the desired features of improved sensor design: a high-density
of active sites, a large surface area, rapid analyte aggregation,
mass transport and a sensitive response with a large linear
range. All these features contribute to its benchmark
Acknowledgements
Z.Y.Z. and S.J.H. are grateful for the PhD fellowship donated
by the China Scholarship Council (CSC). W.L. and S.M.
gratefully acknowledge the Alexander von Humboldt Foun-
dation for their postdoctoral research fellowships. Z.Y.Z. is
grateful to Katia Rodewald and Prof. Bernhard Rieger for
providing the SEM data. We also appreciate the financial
support from Deutsche Forschungsgemeinschaft (DFG) proj-
Table 1: Voltammetric detections of several analytes (belonging to different pollutant classes) on Mn-PCN-222/ITO.
Class
Analyte
Molecular structure[a]
Linear
Limit of
Corresponding
Figure
range [mM] detection [mM]
Catechol (CT)
0.10–100
0.10–100
0.047
0.015
Figure S25
Figure S26
1,4-benzoquinone (1,4-BZQ)
Phenolics and quinone-hydroquinone
Anthraquinone-2,6-disulfonate (AQDS)
Cu2+
0.06–100
0.020
Figure S27
0.06–100
0.05–100
0.30–100
0.013
0.003
0.100
Figure S28
Figure S29
Figure S30
2À
Heavy metal ions
Cr2O7
Cd2+
Ascorbic acid (AA)
0.15–100
0.011
Figure S31
Biological species
Azo dyes
Uric acid (UA)
0.08–100
0.07–100
0.017
0.009
Figure S32
Figure S33
Methyl orange (MO)
[a] Color Scheme: C, grey; O, red; N, blue; H, white; S, gold; Cu, celeste; Cr, yellow and Cd, green.
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Ma, ACS Appl. Mater. Interfaces 2016, 8, 18173 – 18181; d) S.
Biswas, Y. Chen, Y. Xie, X. Sun, Y. Wang, Anal. Chem. 2020, 92,
4566 – 4572.
ect MOFMOX (419949637) and by DFG Priority Program
1928 “Coordination Networks: Building Blocks for Func-
tional Systems” (316376997). Open access funding enabled
and organized by Projekt DEAL.
[10] a) I. Stassen, N. Burtch, A. Talin, P. Falcaro, M. Allendorf, R.
Ameloot, Chem. Soc. Rev. 2017, 46, 3185 – 3241; b) C. H. Su,
C. W. Kung, T. H. Chang, H. C. Lu, K. C. Ho, Y. C. Liao, J. Mater.
Chem. A 2016, 4, 11094 – 11102; c) P. M. Usov, B. Huffman, C. C.
Epley, M. C. Kessinger, J. Zhu, W. A. Maza, A. J. Morris, ACS
Appl. Mater. Interfaces 2017, 9, 33539 – 33543; d) A. L. Semrau,
Z. Zhou, S. Mukherjee, M. Tu, W. Li, R. A. Fischer, Langmuir
2021, 37, 6847 – 6863.
Conflict of Interest
The authors declare no conflict of interest.
[11] a) G. Xu, T. Yamada, K. Otsubo, S. Sakaida, H. Kitagawa, J. Am.
Chem. Soc. 2012, 134, 16524 – 16527; b) G. Xu, K. Otsubo, T.
Yamada, S. Sakaida, H. Kitagawa, J. Am. Chem. Soc. 2013, 135,
7438 – 7441.
Keywords: modular assembly · nitrobenzene detection ·
porphyrinic MOFs · sensors · thin film
[12] S. M. Yoon, J. H. Park, B. A. Grzybowski, Angew. Chem. Int. Ed.
2017, 56, 127 – 132; Angew. Chem. 2017, 129, 133 – 138.
[13] a) Q. Xu, G. Zhan, Z. Zhang, T. Yong, X. Yang, L. Gan,
Theranostics 2021, 11, 1937 – 1952; b) K. Yu, P. Puthiaraj, W. S.
Ahn, Appl. Catal. B 2020, 273, 119059.
[14] G. Granados-Oliveros, E. A. Pꢃez Mozo, F. M. Ortega, C.
Ferronato, J. M. Chovelon, Appl. Catal. B 2009, 89, 448 – 454.
[15] K. S. Walton, R. Q. Snurr, J. Am. Chem. Soc. 2007, 129, 8552 –
8556.
[16] a) P. S. Majumder, S. K. Gupta, Water Res. 2003, 37, 4331 – 4336;
b) R. Emmanuel, C. Karuppiah, S. M. Chen, S. Palanisamy, S.
Padmavathy, P. Prakash, J. Hazard. Mater. 2014, 279, 117 – 124;
c) S. Kubendhiran, S. Sakthinathan, S. M. Chen, P. Tamizhdurai,
K. Shanthi, C. Karuppiah, J. Colloid Interface Sci. 2017, 497,
207 – 216; d) C. Liu, X. Bo, L. Guo, Sens. Actuators B 2019, 297,
126741.
[1] a) S. Liu, C. Lai, X. Liu, B. Li, C. Zhang, L. Qin, D. Huang, H. Yi,
M. Zhang, L. Li, W. Wang, X. Zhou, L. Chen, Coord. Chem. Rev.
2020, 424, 213520; b) D. W. Kimmel, G. LeBlanc, M. E. Me-
schievitz, D. E. Cliffel, Anal. Chem. 2012, 84, 685 – 707; c) B. J.
Privett, J. H. Shin, M. H. Schoenfisch, Anal. Chem. 2010, 82,
4723 – 4741.
[2] a) A. J. Veloso, X. R. Cheng, K. Kerman, Electrochemical
Biosensors for Medical Applications (Ed.: S. Higson), Woodhead
Publishing, 2012, pp. 3 – 40; b) M. M. Rodrꢂguez-Delgado, G. S.
Alemꢃn Nava, J. M. Rodrꢂguez Delgado, G. Dieck Assad, S. O.
Martꢂnez Chapa, D. Barcelꢄ, R. Parra, TrAC Trends Anal. Chem.
2015, 74, 21 – 45.
[3] a) J. F. Rusling, S. L. Suib, Adv. Mater. 1994, 6, 922 – 930; b) N.
Elgrishi, K. J. Rountree, B. D. McCarthy, E. S. Rountree, T. T.
Eisenhart, J. L. Dempsey, J. Chem. Educ. 2018, 95, 197 – 206.
[4] C. C. Wu, T. Yasukawa, H. Shiku, T. Matsue, Sens. Actuators B
2005, 110, 342 – 349.
[5] a) H. Furukawa, K. E. Cordova, M. OꢀKeeffe, O. M. Yaghi,
Science 2013, 341, 1230444; b) H. C. Zhou, J. R. Long, O. M.
Yaghi, Chem. Rev. 2012, 112, 673 – 674; c) R. J. Kuppler, D. J.
Timmons, Q. R. Fang, J. R. Li, T. A. Makal, M. D. Young, D.
Yuan, D. Zhao, W. Zhuang, H. C. Zhou, Coord. Chem. Rev. 2009,
253, 3042 – 3066; d) S. T. Meek, J. A. Greathouse, M. D. Allen-
dorf, Adv. Mater. 2011, 23, 249 – 267.
[6] a) C. S. Liu, J. Li, H. Pang, Coord. Chem. Rev. 2020, 410, 213222;
b) Y. Xu, Q. Li, H. Xue, H. Pang, Coord. Chem. Rev. 2018, 376,
292 – 318; c) S. Kempahanumakkagari, K. Vellingiri, A. Deep,
E. E. Kwon, N. Bolan, K. H. Kim, Coord. Chem. Rev. 2018, 357,
105 – 129; d) S. Tajik, H. Beitollahi, F. Garkani Nejad, I.
Sheikhshoaie, A. S. Nugraha, H. W. Jang, Y. Yamauchi, M.
Shokouhimehr, J. Mater. Chem. A 2021, 9, 8195 – 8220.
[7] a) R. Paolesse, S. Nardis, D. Monti, M. Stefanelli, C. Di Natale,
Chem. Rev. 2017, 117, 2517 – 2583; b) J. Chen, Y. Zhu, S. Kaskel,
Angew. Chem. Int. Ed. 2021, 60, 5010 – 5035; Angew. Chem.
2021, 133, 5064 – 5091; c) H. Lee, K. I. Hong, W.-D. Jang, Coord.
Chem. Rev. 2018, 354, 46 – 73.
[8] a) D. Feng, Z. Y. Gu, J. R. Li, H. L. Jiang, Z. Wei, H. C. Zhou,
Angew. Chem. Int. Ed. 2012, 51, 10307 – 10310; Angew. Chem.
2012, 124, 10453 – 10456; b) Z. Zhou, S. Mukherjee, J. Warnan,
W. Li, S. Wannapaiboon, S. Hou, K. Rodewald, B. Rieger, P. G.
Weidler, C. Wçll, R. A. Fischer, J. Mater. Chem. A 2020, 8,
25941 – 25950.
[17] Y. G. Mourzina, A. Offenhꢅusser, J. Electroanal. Chem. 2020,
866, 114159.
[18] a) A. Palma-Cando, U. Scherf, ACS Appl. Mater. Interfaces 2015,
7, 11127 – 11133; b) Z. Xue, H. Lian, C. Hu, Y. Feng, F. Zhang, X.
Liu, X. Lu, Aust. J. Chem. 2014, 67, 796; c) X. Lu, Y. Quan, Z.
Xue, B. Wu, H. Qi, D. Liu, Colloids Surf. B 2011, 88, 396 – 401.
[19] a) C. W. Kung, T. H. Chang, L. Y. Chou, J. T. Hupp, O. K. Farha,
K. C. Ho, Chem. Commun. 2015, 51, 2414 – 2417; b) S. R.
Ahrenholtz, C. C. Epley, A. J. Morris, J. Am. Chem. Soc. 2014,
136, 2464 – 2472.
[20] a) E. Laviron, J. Electroanal. Chem. 1974, 52, 355 – 393; b) M. B.
Gholivand, M. Khodadadian, Biosens. Bioelectron. 2014, 53,
472 – 478; c) X. Ma, M. Chen, Sens. Actuators B 2015, 215, 445 –
450; d) S. Vinoth, P. M. Rajaitha, A. Pandikumar, Compos. Sci.
Technol. 2020, 195, 108192.
[21] a) S. Dong, G. Suo, N. Li, Z. Chen, L. Peng, Y. Fu, Q. Yang, T.
Huang, Sens. Actuators B 2016, 222, 972 – 979; b) K. Kunpatee, S.
Traipop, O. Chailapakul, S. Chuanuwatanakul, Sens. Actuators B
2020, 314, 128059.
[22] N. Karikalan, S. Kubendhiran, S. M. Chen, P. Sundaresan, R.
Karthik, J. Catal. 2017, 356, 43 – 52.
[23] a) Y. Sang, Y. Cui, Z. Li, W. Ye, H. Li, X. S. Zhao, P. Guo, Sens.
Actuators B 2016, 234, 46 – 52; b) S. Pan, L. Wang, X. Chen, Y.
Tang, Y. Chen, Y. Sun, X. Yang, P. Wan, Electrochim. Acta 2016,
203, 301 – 308.
[9] a) Y. Chen, X. Sun, S. Biswas, Y. Xie, Y. Wang, X. Hu, Biosens.
Bioelectron. 2019, 141, 111470; b) Y. Chen, W. Huang, C. Wang,
X. Zhai, T. Zhang, Y. Wang, X. Hu, ACS Sustainable Chem. Eng.
2020, 8, 13226 – 13235; c) Y. Wang, L. Wang, H. Chen, X. Hu, S.
Manuscript received: June 13, 2021
Revised manuscript received: July 13, 2021
Accepted manuscript online: July 14, 2021
Version of record online: && &&
,
&&&&
&&&&
www.angewandte.org
ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Electrochemical Sensors
Integration of a porphyrinic metal–
organic framework on a conductive
indium tin oxide surface affords an elec-
trochemical sensor, Mn-PCN-222/ITO.
Thanks to detecting several inorganic
ions, organic hazardous substances and
pollutants, Mn-PCN-222/ITO demon-
strates multifaceted sensing with low
detection limits. Benchmark sensing
performances are set not only across
several single components, but also in
the corresponding binary and ternary
mixtures.
Z. Y. Zhou, S. Mukherjee, S. J. Hou,
W. J. Li,* M. Elsner,
R. A. Fischer*
&&&& — &&&&
Porphyrinic MOF Film for Multifaceted
Electrochemical Sensing
Angew. Chem. Int. Ed. 2021, 60, 2 – 9
ꢂ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!