Amino acid-functionalized polyfluorenes: a class of ultra-sensitive fluorescent sensors favoring Cr 2 O 72-†

: Conjugated polymers (CPs) are an intriguing material to build fluorescent Cr 2 O 72-sensors with excellent sensitivity but often lack specific recognition groups. In this study, several typical amino acids with N and O atom identifying groups were incorporated into fluorene and then six polyfluorene derivatives were synthesized using electrochemical polymerization. Compared to other cations and anions, all of these amino acid-functionalized polyfluorenes have good selectivity towards Cr 2 O 72-and enable ultra-trace response with detection thresholds at pM or even fM level.


Introduction
The development of modern industry has caused a large amount of heavy metals to be released into a transitional accumulation of heavy metals in the soil, expanding the area of contamination, thereby affecting the growth of crops and lowering the quality of products.Cadmium ion is more frequently acknowledged as an internationally recognized carcinogen than other heavy metal ions [1].For example, once its content exceeds the standard in water (existing in the form of Cr2O7 2-), it will be enriched in the crops and passed to the human body through the food chain, which will cause various diseases [2,3].Therefore, it is necessary to detect Cr2O7 2-quickly, easily and accurately.
Among the various methods for the detection of Cr2O7 2-, fluorescence analysis has attracted much more attention due to its high sensitivity, simplicity, and rich output signal [4].Conjugated polymers (CPs) have high molar absorption coefficients and strong light absorption abilities, which are good light energy harvesters [5].Moreover, these molecules have a unique "molecular line effect" that can amplify the fluorescence response signal hundreds or thousands of times, which makes them stand out in the fields of biomonitoring and environmental analysis [6,7].Unfortunately, there have been few reports on the specific identification of Cr2O7 2-relying on CPs based fluorescent materials.
Fluorene is a classical blue luminescent polymer material that not only has high fluorescence quantum yield and good optical stability, but also can further improve its photoelectric performance by introducing amino acid groups containing N and O atoms in the side chain [8,9].Amino acids containing N and O atoms are characterized by high ion binding ability, good selectivity, and a strong pH response, which give them certain advantages in the development of efficient and sensitive ion sensors [10,11].Therefore, our group proposed a design strategy, selecting the fluorene as the fluorescence group and introducing amino acid moieties at its C9 position as the recognition groups.Then,

Materials and methods
Six short chains modified with N and O atoms as recognition groups were introduced into fluorene to obtain six monomers (Fmoc-Arg-OH, Fmoc-Glu-OH, Fmoc-GABA-OH, Fmoc-Osu, Fmoc-Ala-OH and Fmoc-Thr-OH) then electrochemical polymerization in the boron trifluoride ethyl ether (BFEE) system (Figure 1).Considering that six monomers cannot be electrochemically polymerized in common organic solvents such as tetrahydrofuran (THF), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) containing supporting electrolytes, an attempt was made to add trifluoroacetic acid (TFA) into BFEE as the electrolyte solution.On one hand, BFEE, as a Lewis acid, has a catalytic effect on the electropolymerization of aromatic compounds [2,3,6,11].On the other hand, the addition of TFA to BFEE leads to the formation of complexes between the monomer and TFA, which reduces the resonance of the aromatic ring and improves the stability of the radical ion while increasing the electrical conductivity [2,3,10,11].

Selectivity of polyfluorene derivatives towards Cr2O7 2-
To explore the recognition capability of Cr2O7 2-, the selective experiment of six polyfluorenes for common anions and cations were studied.It has been experimentally shown that only Cr2O7 2-could quench the fluorescence of six polymers, which indicated these molecules could specific recognition of Cr2O7 2-.It is speculated that the N and O atoms may interact with Cr2O7 2-, which caused the aggregate of the fluorene skeleton and resulted in its fluorescence quenching [2,3,10,11].All the polymers also exhibited good selectivity towards Cr2O7 2-, which was not interfered by other anions and cations.Besides introducing N and O atoms for chelating Cr2O7 2-, another reason for using amino acid groups was to exclude the interference from natural amino acids in agricultural detection.Such good anti-interference ability has been shown in Figure 2, 3.

Sensitivity test of fluorene and polyfluorene derivatives towards Cr2O7 2-
According to the linear relationship tests between the fluorescence intensity of fluorenes and polyfluorene derivatives in DMSO-EtOH solutions and analyzed Cr2O7 2- concentrations.We further explored their sensitivity, and the linear relationship between fluorescence intensity and Cr2O7 2-concentration was studied.As shown in Table 1, monomers 1 and 2 have sensitivity to Cr2O7 2-at the nM level, while the other four monomers only achieve μM.When they were prepared into polymers, their detection sensitivity greatly improved, reaching up to pM and even fM (Table 1), which verifies that the molecular wire effect of polymers can greatly improve the sensitivity of the detection of Cr2O7 2-, which further indicates that this type of amino acid-functionalized polyfluorene fluorescence material has the ability of ultra-trace detection of Cr2O7

Conclusion
In sum, this paper summarizes a design strategy, which constructs a series of amino acid-functionalized polyfluorenes as fluorescent Cr2O7 2-sensors with high selectivity and sensitivity.Thanks to the side-chain groups decorated with N and O atoms, the obtained six monomers and their polymers were not interfered with by common cations, anions, or natural amino acids, which could achieve specific recognition of Cr2O7 2-.In comparison, the polymers showed much higher sensitivity with LODs at the pM and fM levels.This work provides an effective way for the design of high-efficiency fluorescent sensors for Cr2O7 2− even in complex environmental systems.

Citation:
To be added by editorial staff during production.Academic Editor: Firstname Lastname Published: date Publisher's Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.Copyright: © 2022 by the authors.Submitted for possible open access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).six polyfluorene fluorescent derivatives have been successfully fabricated by electrochemical polymerization to construct fluorescence sensors for Cr2O7 2-detection [2,3,10,11].