A number of thorny issues such as the nature of time, free will, the clash of the manifest and scientific images, the possibility of a naturalistic foundation of morality, and perhaps even the possibility of accounting for consciousness in naturalistic terms, seem to me to be plagued by the conceptual confusion nourished by a single fallacy: the old fisherman's mistake.

I.

Once upon a time, an old fisherman used to enjoy the sunset. The sky explodes in fiery colours, the sun descends majestically and dives into the ocean, the sky turns dark blue and one by one the stars light-up.

But a man from the city came along and told the old fisherman: "You know: the sun does not really dive into the ocean. It stands still out there and is always shining. What you see is only a perspectival show due to our movement."

The old fisherman was stunned. He trusted the man from the city and began to worry. The sunset is an illusion. Hence it is not real. He had been watching a non-existing event for years. He was deluded all his life.

If the sunset is an illusion -he thought-we cannot rely upon it. We have to learn to think without sunsets. He tried, and it was a disaster: he did not know when to go to sleep anymore, he did not expect a sunset in the evening, and if he saw one, he repeated to himself: "It is an illusion, it is not real, there is no sunset, the sun never dives into the ocean: the sun is always shining, I should take this seriously, I should not go to sleep." So, he couldn't sleep anymore; he lost his mind.

The old man was obviously making a mistake. It seems a simple one, but it is a subtle one. I find precisely this same mistake repeated over and over in a number of domains. I think that this mistake confuses the discussion on a number of thorny and much debated issues. I describe what precisely the mistake is, and I suggest that recognizing it could make a number of those issues a bit less troublesome.

II. THE FALLACY

The question bothering the old fisherman is whether the sunset is real or illusory. On the one hand, the reality of the sunset is denied by the knowledge of the city man, which the old fisherman trusts. On the other hand, denying the reality of the sunset appears ridiculous and leads the old man to absurd and dramatic deductions. Where is the catch?

The catch is in the meaning of the concept "sunset". The old man grew up with a clearcut notion of what a sunset is:

(a) The sunset is the diving of the sun into the water of the ocean.

This, for the old fisherman, was what a sunset is. Its very nature. Its definition. The very meaning of the concept "sunset". A careful conceptual analysis of his notion of sunset would have identified it with the dive of the sun into the ocean. When the old fisherman is told that the sun does not dive into the ocean, the unavoidable conclusion is that there is no sunset.

But the rest of us, who know our Copernicus, are still very happy to talk about sunsets. We rely upon sunsets, we enjoy sunsets, and it would not even cross our mind to state that there is no sunset in our Universe. Why?

Because -here is the key point-we have reconceptualized the notion of "sunset" adjusting it to our increased knowledge. Say: (b) A sunset is the collection of real phenomena happening around us when the rotation of the Earth moves us out of the lit part of the globe.

Let us assign different names to these two definitions: (a) defines "sunset-(a)" while (b) defines "sunset-(b)". The knowledge of the man of the city denies the reality of sunset-(a), but not the reality of sunset-(b). So, a reasonable reaction by the old fisherman should have been something like: "Oh gosh, I always thought that the sunset I used to watch was a sunset-(a), but this is not true. The sunsets I saw were all sunsets-(b)." But sunsets-(b) provide the very same vision of the sun majestically descending towards the ocean, the sky exploding in fiery colours, the sky turning dark blue and one by one the stars lighting up and so on. Furthermore, sunsets-(b) equally stir emotions, they equally tell the time to go to bed, they are as reliable in their recurrence as sunsets have always been. Hence the notion of sunset can play exactly the same role as before in the fisherman's life, even if according to his strict definition (a) there are no sunsets at all.

Summarizing: there is a concept ("sunset") that plays an important role in the old fisherman life. He naturally understands this notion as sunset-(a). But sunsets-(a), he finds out, do not exist in nature. However, sunsets-(b) do exist, and they play precisely the role that sunsets have always played all along his life. No trouble follows in replacing the definition (a) with the definition (b), and using the notion of sunset just as before. Believing that there is trouble is the mistake that the old fisherman makes.

Let's draw the general lesson from the parable. There are concepts that play an important role in our dealing with the world. We can have a good conceptual understanding of them. We know what they mean. But it happens that increased knowledge about the structure of the world shows us that our understanding of the phenomena they designate contains something incorrect. However, there is a modification of the same concept that does play precisely the role as the original concept played. No trouble follows in replacing the old understanding with the new one.

Believing there is trouble is the old fisherman's mistake.

In the next section I illustrate how precisely this mistake plays out in a number of current debates. Before getting to that, however, one more observation may be useful. The account above may not capture the full story yet. Some old fishermen are stubborn guys. Our old fisherman may well end up understanding all of the above, but still feel a sense of loss. After all, he was emotionally attached to the idea of the sun actually diving into the water. He could almost visualize the splashes and the boiling around the sun immersing into the waves. So, he may mourn the loss of sunsets-(a). This might be psychologically comprehensible, and we would be culturally insensitive to deny him the right to his loss. We may feel sorry for him, and perhaps even empathize a bit with him. This is comprehensible. But if the old man fails to see that sunsets-(b) are real and if he denies that they are fully entitled to play the vast amount of functions that "sunset" used to play for his life, then he is clearly just mistaken.

III. EXAMPLES

In the following, I list a number of issues that are debated today, sometimes fiercely. I do not pretend to offer conclusive solutions to all of them, perhaps nor even to say anything particularly original about them. My point is to notice what seems to me to be a common fallacy in the way all these issues are sometimes addressed.

Needless to stay, not all the authors involved in these debates fall in this fallacy. The fallacy is well recognized by some players in each of the debates. My aim here is to point out that it is the very same fallacy that creates confusion in each of these issues. This may make it easier to avoid it.

A. Free will

We have the distinct experience of being able to make decisions that are free: they depend on us. Moral responsibility, legal theory, and our own understanding of ourselves rely upon the idea that the source of some decisions is the individual, who therefore bears responsibility for the decisions taken. A naive way we can think about the nature of free will is the following:

(a) We have the power to steer the future along different paths, in a way that is not due to random chance, and without different future paths implying any difference at all in the past state of the world (including ourselves).

Call this "free-will-(a)". Now, the man from town, incarnated into modern physics, tells us that free-will-(a) is incompatible with the laws of nature. In the classical limit, different futures require different pasts. In quantum physics, the past fully determines the probability distributions of future events.

Like the old fisherman, we panic. If there is no free will, our idea of ourselves crumbles. We agonize. We oscillate between trusting science or remaining faithful to our deep intuition about our freedom. We fear the scientific advances to jeopardize the very foundation of our legal system and our morality.

It is the same panic as the old fisherman's. Consider a possible definition of free-will-(b). Say:

(b) At a given state of the external world, with given external stimuli and given internal memories, our future actions are still not determined. They are affected by complex mental process, carried on by the physical system that we are.

Nothing in modern science denies the existence of free-will-(b), of course. (See for instance [Dennett 1984]). Free-will-(b) is perfectly sufficient to underpin our sense of internal freedom, our legal system and our morality; it justifies our sense that it is us who decide, and it accounts for all phenomena that we traditionally attribute to free will.

For instance, to the extent legal theory requires a notion of free choice, free-will-(b) is largely sufficient. We incarcerate persons on the basis of the assumption that their mental processes have led to crime in contexts and under circumstances where other people would not have done the same. This notion of choice is coherent with free-will-(b). Whether we hold a dissuasive, a punitive, a redeeming, or a vengeance motivation for incarceration, in all these cases, the reality of free-will-(b) suffices not only to define legal responsibility, but also provide the rational motivation for incarceration in the concrete cases in which free choice is invoked. (I touch on moral responsibility later on.) Furthermore, it is also fully comprehensible why our (free, in the sense just clarified!) decisions have lead us to establish laws accordingly. Hence, there is no contradiction between any of this and modern science. To think that there is a contradiction is the old fisherman mistake.

Of course there is more. Remember that the old fishermen, stubborn guy, could still feel a sense of loss at the idea that the sun is not actually diving in the ocean with a splash. In the same vein, some of us, stubborn guys, may feel a sense of loss for free-will-(a), namely for the loss of the idea that something in us is external to the natural realm, and can influence it from the outside. This fact may be hard to swallow for some. During the Renaissance, some found it hard to swallow the loss of an immovable Earth. To this, I see no remedy but mourning.

But the point I am making is a different one: there is no contradiction between the reality of the phenomena we commonly denote "free will", with the full use we make of this concept, including in our psychology, morality and law, and the discoveries of science. The reason to think that there is a contradiction is the old fisherman fallacy: failing to distinguish a rigid (mistaken) understanding of a concept, from the actual (fluid) role that it plays within our overall conceptual structure.

To learn is to adjust our concepts, not to be enslaved by the way we intend them at some moment of time.

B. Manifest and scientific image of the world

The example of the old fisherman is a special case of the supposed tension between the "manifest" and the "scientific" image of the world [Sellars 1962]. A piece of matter appears to be continuous, while science tells us that it is mostly empty and just scattered with atoms. The ground appears static, while the Earth is spinning.

The clash between the two images is easily resolved in the way illustrated above: realizing that that the manifest image is neither "illusory", nor in contradiction with the underlying complexity revealed by scientific research.

The error is to mistake an appearance for a definition. We have analogous situations in everyday life: seen from a distance, a forest along the side of a mountain appears as a uniform velvet green. This is a "manifest" image of a forest. But if I walk into the forest, I see trees, trunks, leaves, insects -a rich complexity. Does this make the "uniform velvet green" illusory? Does this create a conflict? It does not. Things look one way seen from a distance and another way seen from close. The "scientific image of the world" does not clash with the manifest image, it is a more detailed perspective on the same objects. It does not falsify the manifest image: it grounds it, like the perspectival understanding of a sunset explains the image of the sun diving into the ocean.

Once again, it is the analysis of the meaning of our words that can be misleading. If we define a table as (a) a continuous piece of wood, we are posing continuity to be part of the conceptual definition of matter. Then we get endlessly confused in learning atomic theory. But if we are flexible in redefining what we mean by continuous matter at the light of the advances of science between the XIX and the XX century, there is no tension between the continuous surface of a table and its atomic structure. A table is simply (b) an atomic structure that is continuous at scales much larger than the atomic scale.

The mistake of believing there is a clash is to take the manifest image as the definition of reality, instead of taking it, as we should, as a provisional, approximate, effective, useful level of conceptualization of reality, to be kept flexible in view of the development of knowledge.

C. The nature of time

I focus here on one specific aspect (among the many) of the debate on the nature of time: orientation, or "the arrow of time". The macroscopic world we experience is time-oriented. We witness phenomena that we never see happening in time-reversed order. Hence our naive intuition about time is that it is necessarily oriented. Here is a possible definition of what time is:

(a) Time is an oriented flow.

However, science has discovered that all elementary evolution equations are time reversal invariant: if a phenomenon happens, the phenomenon obtained by time reversing it is allowed by these equations to happen as well [Price 1996].

If, like the old fisherman, we think that time is by nature oriented (as in definition (a) above), we are confused by finding out that irreversibility is a contingent fact regarding a macroscopic description of events, due to entropy happening to be low in the past. If we hold on to definition (a), we deduce that there is no time. We panic. We see a dramatic clash between microphysics and experience. Shall we believe in microphysics and declare that our entire experience of the unidirectional flow of time is illusory? Should we hold on to the cogency of our experience and alter the microphysics? Should we declare current physics incomplete, because it does not include time-(a) in its foundations? None of the above is necessary. We are making the old fisherman's mistake. We fail to recognize that we are indicating two different notions with the same name. There are a more refined notion of time useful to describe elementary dynamics, where time is not fundamentally oriented, while orientation appears only in the macroscopic account of phenomena. Say:

(b) Time is an independent parameter in the equations of motion, in terms of which the macroscopic account of physical phenomena in our universe turns out to be irreversible.

Time-(b) accounts for our experience, hence justifies our intuition, underpins all irreversible phenomena of the world, and is compatible with current microphysics. There is no tension.

Once again, some of us, stubborn guys, will keep mourning for the lost notion of a fundamental oriented time. Ok for the mourning. But to argue that a fundamental orientation of time (independent from the macroscopic approximation) is required to make sense of our experience of the word, is like arguing that the sun truly splashing into the ocean's waves is required for the sunset we see.

D. The possibility of a naturalistic foundation of values

I touch a wider theme: the possibility of a naturalistic foundation of values, such as moral ones. Values can be held and studied by themselves and as such. But in the context of a naturalistic worldview, we can also ask for an understanding of how they relate to -say -biology, natural evolution, cultural evolution, or similar [Darwin 1871]. In this context, one considers the natural origin of values. In this context they are not literally undersigned by God, they are not absolute Kantian imperatives, they are not valid by their abstract nature.

Here I am not concerned with the extent specific accounts of the natural origin of values are convincing. Rather, I am interested in a surprisingly widespread reaction to any such account: the idea that it empties the significance or the cogency of the values themselves, either because it relativizes them, or because it undercuts their supposed foundation. The point I want to make here is only that this reaction is another example of the old fisherman's mistake.

To put it pictorially, let me consider a wiser man than the fisherman of the initial story, one that does not panic so easily. This wise man loves his children and helps the poor. When asked why he does so, he answers: because this is what God wants from me. But one day the man from the city ends up convincing him that the reason he loves his children and helps the poor is because of his biology.

The old man listens carefully, thinks for awhile and then says: "It may be so, as you say, but I still love my children, and I still help the poor."

What has happened to him? He has simply allowed for the possibility of re-conceptualizing a real phenomenon, his morality, by changing his definition from: The denial of the reality of (a) does not imply that the phenomenology of love and charity is false or ill founded, mistaken or illusory. It remains real, because it can be re-conceptualized, due to a change of general cultural context. Whether (a) or (b) are more compelling depends on the cultural context, of course.

If in the same situation the wise man had panicked: "Oh Gosh, if there is no God telling me what to do, there is no morality", he would have committed the old fisherman's mistake. This is the reaction illustrated by Dostoyevski in a renown passage of Brother Karamazov, which well represents the entire (misplaced) rhetoric against the supposed XX century "nihilism". The mistake is to be stubbornly anchored to an interpretation of a rich phenomenologymorality-in terms of a strict definition of this phenomenology that relies on incomplete knowledge.

The remarks above allow us to consider the question whether moral responsibility (as opposed to legal responsibility) depend on freewill-(a). Is it put in jeopardy by the determinism of classical physics or the random probabilism of quantum physics? As mentioned above, we can consider values, including moral values, as such, or study the coherence of the ensemble of moral phenomena with a naturalistic world view. In the first case, there is nothing wrong assuming free-will-(a): we are in the context of an incomplete account or reality, and within this account choice can be coherently taken as literarily un-caused. If instead we enquire about the compatibility between free will and modern science, we are doing something else: we are placing [Price 2011] the moral discourse within a naturalistic worldview. Once again, there is no contradiction, because any naturalistic account of morality refers to free-will-(b), not to free-will-(a). The confusion comes only if we pretend the second to remain valid outside the context where we can use it.

E. Consciousness

Finally, I find that the same fallacy informs the debate about consciousness. Chalmers' well known distinction between the "easy" and the "hard" problem of consciousness is rooted in this fallacy [Chalmers 1996].

There are substantial differences with the situation of the old fisherman, and the previous cases. In the case of the sunset, for instance, we have the "easy" problem fully solved: Copernican theory. In the case of consciousness we are far from an equally solid understanding of the phenomena.

But the recurring claim that there necessarily is a "hard" problem, over and above the unsolved "easy" problem, is to base the very formulation of the issue on a conceptual analysis what it means for us to be conscious. Our intuition is that -whatever the phenomena-to be conscious must be something over and above whatever process is happening in the brain.

Chalmers' key argument, for instance, is the zombie argument: the same brain processes that happen together with a subjective experience can also be conceived to happen in the absence of this subjective experience. But why could one conceive this to happen? One can conceive this to happen only if one holds on to a conceptual understanding of ourselves as entities independent from the physical processes in the brain ("consciousness-(a)"). It is the presumed understanding that implies that we can conceive the separation between brain and mind.

A claim is made that we know directly the intrinsic nature of consciousness by being conscious, so we couldn't be mistaken about its nature. Centuries of advances in knowledge teach us that there are no intuitions we can't be mis-taken about. This, I believe, is precisely what naturalism is all about It may well be true that consciousness-(a) is incompatible with the current naturalistic account of the world, but this is no argument against the possibility that the "easy" problem could lead to an understanding of brain processes where a "consciousness-(b)" concept can account for the full experience of subjectivity.

In simpler words, the intuition that authors like Chalmers of Nagel [Nagel 2012] defend is just what it is: an intuition promoted to a definition, or a certainty. Like the intuition of the old fisherman, also promoted to a definition, that the very nature of sunset is the sun splashing into the ocean.

Intuitions turn out to be wrong. Especially when they find themselves in ever-increasing contrast with everything we are learning about the natural world.

IV. CONCLUDING REMARKS

The old fisherman's mistake is failing to understand that strict definitions based on a careful conceptual analysis of the way we think and the way we speak become misleading once knowledge increases.

In a sense, all this is trivial. After all, if we just define things carefully, everything goes in order. Sunset-(a) has a different definition from sunset-(b); free-will-(a) has a different definition from free-will-(b) and so on. The mistake is the blindness to these distinction.

The subtle point is that a complex phenomenology (sunset, free will, flowing of time, consciousness,...) is commonly named, then rigidly characterized by a concept. But the concept so generated does not refer solely to the phenomenology: it also refers to an understanding we have of this phenomenology, which at the light of increased knowledge turns out to be wrong. The confusion comes because we confuse the part of the definition that refers to the phenomenology from the part that depends on our wrong beliefs.

There is no pure phenomenology independent from our general conceptual structure. The other way around, it seems to me that we have learned, at least since Quine, if not since the second Wittgenstein, that there is no concept that may not be challenged by empirical knowledge.

The historical growth of knowledge has repeatedly shown us that concepts that seemed "clear and distinct", and appropriate, can in fact turn out to be inadequate. The pre-relativistic concept of "simultaneity", or the pre-Copernican concept of "motion" turned out just to be inadequate.

The moral is that to understand is to recognize that we have to modify concepts, not only to clarify and combine them. Howard Stein expresses this point beautifully [Stein 2004]: "This mistake is the assumption that a clarification of 'ideas', or concepts, should always or can always -precede the advance of knowledge." And: "the two enterprises, that of knowledge and that of understanding, are inextricably intertwined". Where " 'understanding' here refer [s] to the grasp of ideas, or concepts". The formulation of the useful concepts may come after knowledge, not before it, because "old" concepts may be internally plagued by wrong knowledge.

Once this is clear we have always two options. The first option is to keep a name attached to the inadequate comprehension and say, for instance, that there is no sunset, there is no free will, there is no time, there is not table, there is no consciousness: these are "illusions". This is correct, if we take these names to indicate their (a)-version.

But there is also the second alternative: there are sunsets, there is free will, there is time, there are tables, there is consciousness; they are just what their (b)-version implies.

What we decide to indicate by a world is conventional, and whether we take the first or the second option is a matter of convenience, not substance. (If I thought that the shadows in my room where ghosts, and then realize they were casts by branches outside the window, I may prefer to drop the ghost language altogether and call it illusory, rather than reinterpreting "ghosts" as "shadows of the branches".) But to believe that there is a problem is the old fisherman mistake.

This implies that (taking the second option) concepts that we use can play the full spectrum of their role, and still denote what they used to denote (sunsets, forests, pieces of matter, oriented time, values, moral imperatives, subjective experience,...), even if we change our understanding of what they actually refer to, namely if we redefine them taking into account increased knowledge. We got confused because we took them rigidly.

"But this is not what I mean by sunset!" answers the old fisherman to anybody trying to talk him out of this confusion by illustrating what sunsets really are.

"But this is not what I mean by free will, by time, by morality, by consciousness" answers the philosopher who resists what science is teaching us about the nature of time or the functioning of ourselves. And this answer is exactly right. The problem is that to remain anchored to "this is what I mean by" is to refuse to take away the screen that blinds us from the comprehension of the actual phenomena that we witness.

It is not the nature of the real sunset to have the sun splashing in the water, it is not the nature of our actual freedom to be independent form the natural order, it is not the nature of natural time to be necessarily oriented, it not in the nature of real continuous matter to be continuous at any scale, it is not in the nature of the actual moral behaviour of people or in the nature of actual subjective experience to be unaccountable in terms of known elementary laws.

Using this language, we can say that it is not that sunsets, free will, tables, consciousness, the oriented flow of time, etcetera are "illusory". They are perfectly real. What is an illusion is that a sunset is literally the sun diving in the waves, free will is literally an external intervention above physical evolution, and so on. Which is how we (mistakenly) might "intend" these phenomena.

Conceptual analysis in the sense of precise clarification of what we now intend by the concepts we use can become dangerously mislead-ing, as soon as we learn something new.

The reason is that knowledge is not just learning new facts about our old concepts. It is often to realise the inadequacy and misleading character of some of our old concepts, and the intuitions that ground them.

I find surprising that a single specific fallacy nourishes the confusion around so many issues. *** I am deeply indebted to Jenann Ismael for enlightening exchanges and comments, from which several of the ideas presented in this paper have originated. This work was made possible through the support of the FQXi Grant FQXi-RFP-1818 and by the QISS grant ID# 60609 of the John Templeton Foundation.

-Chalmers, David, 1996, "The Conscious Mind". Oxford: Oxford University Press.

- Darwin, Charles, 1982[1871], "The Descent of Man, and Selection in Relation to Sex". Princeton: Princeton University Press.

-Nagel, Thomas, 2012, "Mind and Cosmos: why the materialist neo-Darwinian conception of nature is almost certainly false". Oxford, New York: Oxford University Press.

-Dennett, Daniel, 1984, "Elbow Room: The Varieties of Free Will Worth Wanting". Cambridge, MA: MIT Press.

-Price, Huw, 1996, "Time's Arrow and Archimedes' Point: New Directions for the Physics of Time" Oxford: Oxford University Press.

-Price, Huw, 2011, "Naturalism without mirrors". Oxford: Oxford University Press.

-Sellars, Wilfrid, "Philosophy and the Scientific Image of Man," in Frontiers of Science and Philosophy, Robert Colodny ed. Pittsburgh: University of Pittsburgh Press, 1962: 35 -78. -Stein, Howard 2004, "The Enterprise of Understanding and the Enterprise of Knowledge " Synthese 140: 135 -176, 2004.

  (a) Love and charity are what God asks me to do, to (b) Love and charity are what I do because of my nature and culture.


Khawla Mili

Zouhour Hsine

Yves Chevalier
email: yves.chevalier@univ-lyon1.fr

Souhaira Hbaieb

Rym Mlika
email: mlikarym@yahoo.fr

ZnS quantum dots as fluorescence sensor for quantitative detection of tetracycline

Keywords: ZnS quantum dots, 3-Mercaptopropionic acid, 3-Mercapto-1-propanesulfonic acid, Fluorescence quenching, Tetracycline detection

X-ZnS quantum dots (QDs) are synthesized in aqueous solution by a simple aqueous chemical route where X represents 3-mercaptopropionic acid (MPA) or 3-mercapto-1-propanesulfonic acid (MPS) which are two well-known low cost and stable ligands. The synthesized X-ZnS QDs yielded aqueous suspensions of nanoparticles with excellent monodispersity, water solubility and fluorescence stability with a relatively high fluorescence quantum yield of 11 % and 10 % for MPA-ZnS QDs and MPS-ZnS QDs respectively. Under optimal conditions, the prepared X-ZnS QDs are used to detect tetracycline (TC) based on their fluorescence quenching induced by the target analyte via electrostatic interaction. The Stern-Volmer-type equation has been fitted to the quenching curve of each QDs, from which MPA-ZnS QDs has shown a larger linear range and a lower limit of detection than MPS-ZnS QDs. The fluorescence response of MPA-ZnS QDs is linearly with respect to TC concentration over a wide range from 200 nM to 6000 nM with a detection limit of 30 pM. The combination of non-toxicity, simplicity and low cost together with high analytical performance of the proposed MPA-ZnS QDs make it promising for the determination of trace TC in food products of animal origin.

I. Introduction

Tetracycline (TC) is a type of broad-spectrum antibiotic that has various applications. Apart from the treatment of human diseases, TC is widely used in veterinary medicine as a veterinary drug and a food additive for the treatment of animal infections and animal growth promoter thanks to their broad-spectrum antimicrobial, good oral absorption, and low cost [START_REF] Qu | Direct and indirect fluorescent detection of tetracyclines using dually emitting carbon dots[END_REF]. However, the abuse leads to TC residues in animal-derived foods such as meat, milk, honey, eggs and chicken. Consequently, the intake of food with antibiotics residues for a long-time actually is a normal continual use of low dose TCs, which increases the resistance of pathogens to antibiotics and results in severe threat to health [START_REF] Qu | Direct and indirect fluorescent detection of tetracyclines using dually emitting carbon dots[END_REF][START_REF] Zhao | Functionalized carbon quantum dots as fluorescent nanoprobe for determination of tetracyclines and cell imaging[END_REF][START_REF] Liu | Recent advances in sensors for tetracycline antibiotics and their applications[END_REF] such as liver injury, tooth yellowing, bacterial resistances, allergic reactions and intestinal flora disorder [START_REF] Liu | A novel fluorescence probe for rapid and sensitive detection of tetracyclines residues based on silicon quantum dots[END_REF]. For these reasons, several control authorities set maximum residue limit (MRL) for TCs in foods; e.g., 0.3 ppm for the US Food and Drug Administration (FDA) and 0.1 ppm for the European Union and Codex Aliment Arius [START_REF] Muhammad | Surface-enhanced Raman spectroscopy for trace detection of tetracycline and dicyandiamide in milk using transparent substrate of Ag nanoparticle arrays[END_REF][START_REF] Du | Magnetic stir cake sorptive extraction of trace tetracycline antibiotics in food samples: preparation of metal-organic framework-embedded polyHIPE monolithic composites, validation and application[END_REF]. Therefore, the quantification of trace TC in animal products is extremely important. To determinate low levels of TC, numerous traditional analytical strategies have been applied in the determination of antibiotic residues, such as high-performance liquid chromatography coupled with an ultraviolet detector (HPLC-UV), liquid chromatography-tandem mass spectrometry (LC-MS/MS) and enzyme-linked immunoassay (ELISA). However, these highly sensitive methods [START_REF] Chen | Label-free fluorescent strategy for sensitive detection of tetracycline based on triple-helix molecular switch and G-quadruplex[END_REF] are costly, time-consuming, and entail sophisticated equipment with well-trained staff [START_REF] Xu | Metal-enhanced fluorescence detection and degradation of tetracycline by silver nanoparticle-encapsulated halloysite nano-lumen[END_REF]. Furthermore, these methods require complex sample pretreatments involving severe reaction conditions [START_REF] Liu | A novel fluorescence probe for rapid and sensitive detection of tetracyclines residues based on silicon quantum dots[END_REF][START_REF] Hou | Rapid microwave-assisted synthesis of molecularly imprinted polymers on carbon quantum dots for fluorescent sensing of tetracycline in milk[END_REF][START_REF] Elmizadeh | Fabrication and optimization of a sensitive tetracycline fluorescent nano-sensor based on oxidized starch polysaccharide biopolymercapped CdTe/ZnS quantum dots: Box-Behnken design[END_REF]. In addition, the large size of these instruments compromise carry out for in-situ measurements [START_REF] Lin | Europium(III) modified silicone nanoparticles for ultrasensitive visual determination of tetracyclines by employing a fluorescence color switch[END_REF]. Fluorescence sensing approaches based on changes of spectral fluorescence resulting from the interaction between sensing material and analyte provide an elegant strategy to overcome most of such difficulties [START_REF] Duan | Facile synthesis of N-acetyl-l-cysteine capped ZnS quantum dots as an eco-friendly fluorescence sensor for Hg 2+[END_REF]. This sensing system is practically simple, low-cost, rapid, suitable for on-site detection and exhibits adequate performance for biological samples [START_REF] Zhang | A novel fluorescent "turn-on" aptasensor based on nitrogendoped graphene quantum dots and hexagonal cobalt oxyhydroxide nanoflakes to detect tetracycline[END_REF][START_REF] Sharma | Rapid optical sensor for recognition of explosive 2,4,6-TNP traces in water through fluorescent ZnSe quantum dots[END_REF]. In recent years, water-dispersible quantum dots (QDs) have been widely used as fluorescent nanomaterials probes for fluorescence sensors, thanks to their numerous advantages that can provide the required sensors with the desired features [START_REF] Sharma | Rapid sensing of lead metal ions in an aqueous medium by MoS2 quantum dots fluorescence turn-off[END_REF][START_REF] Sharma | Synthesis of photoactivated highly fluorescent Mn 2+ -doped ZnSe quantum dots as effective lead sensor in drinking water[END_REF][START_REF] Diaz-Diestra | L-cysteine capped ZnS: Mn quantum dots for room-temperature detection of dopamine with high sensitivity and selectivity[END_REF]. Their size, tunable absorption and emission spectra, high stability, large fluorescence quantum yields, narrow spectral bands, negligible photo-bleaching and extensive surface modification make them excellent fluorescent indicators for chemical and biological assays [START_REF] Ben Brahim | Interaction of l-cysteine functionalized CdSe quantum dots with metallic cations and selective binding of cobalt in water probed by fluorescence[END_REF][START_REF] Yan | Carbon dots synthesized at room temperature for detection of tetracycline hydrochloride[END_REF]. Recently, a variety of fluorescent sensors of TC based on the use of QDs have been reported in literature.

Indirect detection relying on the fact that Eu 3+ or Al 3+ can bind specifically with TC has been reported [11,[20], however these sensors were found to be complex [START_REF] Liu | One-pot carbonization synthesis of europium-doped carbon quantum dots for highly selective detection of tetracycline[END_REF] and exhibited a narrower linear range of TCs compared to direct sensors [START_REF] Qu | Direct and indirect fluorescent detection of tetracyclines using dually emitting carbon dots[END_REF]. Aptamers-based fluorescent sensors have been designed for the detection of TC [START_REF] Wang | Silica-coated silver nanoparticles decorated with fluorescent CdTe quantum dots and DNA aptamers for detection of tetracycline[END_REF]. Nevertheless, despite their high affinity, these approaches involve tedious DNA labeling or modification techniques based on condensation reactions between the carboxyl group and the amino group, or streptavidin-biotin specific reaction, which make these processes technically demanding, time consuming, expensive in terms of reagents that may also affect the affinity of the aptamer [START_REF] Liu | A label-free fluorescence detection strategy for lysozyme assay using CuInS2 quantum dots[END_REF]. Other fluorescent sensors based on quantum dots combined with molecularly imprinted polymers (MIP) as recognition elements have been used for the detection of TC [START_REF] Zhang | Fluorescence probe based on hybrid mesoporous silica/quantum dot/molecularly imprinted polymer for detection of tetracycline[END_REF]. However, these sensors still have shortcomings, especially their slow response and limited chemical stability, which lead to low sensor rate and lifetime [START_REF] Wang | Surface-imprinted fluorescence microspheres as ultrasensitive sensor for rapid and effective detection of tetracycline in real biological samples[END_REF]. Based on all these limitations, a simple, direct, highly sensitive and label free fluorescent sensor for trace determination of TC is highly recommended. Recently, a fluorescent sensor that meets all these expectations based on capped CdS quantum dots has been used for the quantification of TC [START_REF] Anand | Interaction of tetracycline with l-cysteine functionalized CdS quantum dots -Fundamentals and sensing application[END_REF]. However, utilization of Cd/Se/Te-based QDs is restricted because of their strong cytotoxic activity [START_REF] Chang | Mercaptophenylboronic acid-capped Mn-doped ZnS quantum dots for highly selective and sensitive fluorescence detection of glycoproteins[END_REF][START_REF] Cao | Facile synthesis of chitosancapped ZnS quantum dots as an eco-friendly fluorescence sensor for rapid determination of bisphenol A in water and plastic samples[END_REF]. Thus, Zn-based QDs may be regarded as a promising new choice of fluorescent probe for the detection of TC, as they are a type of nontoxic (or low-toxic) QDs compared with traditional Cd/Se/Te-based QDs. To the best of our knowledge, there are no reports on fluorescence detection for TC with Zn-based QDs.

Herein, considering the high toxicity of Cd, we developed a simple, efficient and eco-friendly fluorescence sensor for TC based on ZnS QDs whose surface was modified with mercaptopropionic acid (MPA) or 3-mercapto-1-propanesulfonic acid sodium (MPS) yielding the formation of stable MPA-ZnS QDs and MPS-ZnS QDs respectively. The surface modification of ZnS QDs with non-toxic and low cost MPA and MPS allowed at the same time to passivate and solubilize quantum dots and to make their surface susceptible to ionic interactions with the target material. The concentration of NPs as well as the pH of the electrolyte leading to a good electrostatic interaction between the TC and the X-ZnS QDs have been studied.

Experimental

Reagents

Zinc acetate di-hydrate (Zn(CH3COO)2.2H2O, 98 %), sulphide nonahydrate (Na2S.9H2O, 98 %), 3-mercaptopropionic acid (MPA, C3H6O2S), 3-mercapto-1-propanesulfonic acid sodium salt (MPS, C3H7NaO3S2, 97 %), sodium hydroxide (NaOH, ≥ 97 %), and tetracycline (C22H24N2O8, ≥ 95 %) were purchased from Sigma-Aldrich. Distilled water was used as the solvent for all steps of the ZnS QDs synthesis. All chemicals were used as they were purchased without any further purification.

Synthesis of X ligands capped ZnS QDs

The ZnS QDs in presence of the various capping agents such as MPA and MPS were prepared according to the reported procedures in literature [START_REF] Nguyen | Energy transfer in poly(vinyl alcohol)-encapsulated Mn-doped ZnS quantum dots[END_REF][START_REF] Li | A room-temperature phosphorescence sensor for the detection of alkaline phosphatase activity based on Mn-doped ZnS quantum dots[END_REF]. First, 0.1 M of Zinc acetate dihydrate and 14.3 mmol of X-capping agent in 50 mL of water were dissolved together where the X ligands act as a capping agent to prevent the agglomeration of particles and stabilize the ZnS NPs. After that, the pH value of the mixed solution was adjusted to 11.20 by 2 M NaOH solution and stirred for 30 min on a magnetic stirrer to get a homogeneous solution. Then, 25 mL of sodium sulfide have been poured drop by drop into zinc acetate and X ligand solution under continuous stirring at 100 °C for about 2 h in the presence of nitrogen gas. When the sodium sulfide solution was added, a milky mixture was formed indicating the formation of zinc sulfide nanoparticles. After the reaction was completed, a white colored precipitate was obtained which was separated by centrifugation. For purification, the obtained ZnS NPs were precipitated with ethanol, separated by centrifuging at 4000 rpm then dried in a desiccator under vacuum. The precipitate has been dried in an oven at 80 o C for 4 h to get powder of X capped-ZnS samples.

The synthesis of X capped-ZnS samples is summarized in Fig. 1.

Fig. 1. The schematic illustration of the synthesis of X-ZnS QDs

Instrumentation

The UV-visible absorption spectra were recorded with a Shimadzu UV-310PC spectrophotometer in the range of 200-600 nm.

Photoluminescence experiments were carried out on a Cary Eclipse fluorescence spectrometer and the spectra were collected in the range of 300-600 nm with an excitation at 300 nm.

The FTIR spectra of QDs in solid state were recorded with Nicolet Is50 FTIR spectrometer in the range of 400-4000 cm -1 .

The surface charge changes upon pH variation of the samples were monitored through zeta potential determination by means of electrophoresis measurements using a Zetasizer NanoZS (Malvern, UK) instrument.

X-ray diffraction (XRD) powder spectra were recorded by XPERT-PRO-MPD Panalytical Xray diffractometer using the Cu Kα emission (λ = 1.5405 Å) as the incident radiation. The scanning range was between 20 ° and 70 ° by steps of 0.02 °.

The morphology and particle size of ZnS samples were determined by means of a JEOL JEM 1220 transmission electron microscope operating at 120 kV. The samples were dispersed in water, deposited onto a thin Formvar film supported on a Cu grid, and dried in open air before transmission electron microscopy (TEM) observation.

General procedure for the detection of TC molecules

To study the quenching effect of TC on the fluorescence (FL) intensity of X-ZnS QDs, 500 mL of X-ZnS QDs solution (1.5 mmol•L -1 ), 200 mL of 0.1 mol•L -1 PBS (pH 7.8) and different amounts of TC were successively added into a 2 mL calibrated test tube, diluted to the mark with deionized water, shaken thoroughly and equilibrated for 2 min until the solution was fully mixed. The fluorescence spectra were recorded from 200 nm to 600 nm, and the fluorescence (FL) intensity of the maximum emission peak was used for quantitative analysis.

Results and discussion

Morphology and crystallography of quantum dots

The morphology and the particle size of the synthetized X-ZnS QDs were examined by transmission electron microscopy and shown in Fig. 2. It can be seen that X-ZnS QDs exhibit approximately spherical shapes, good dispersity with uniform sizes of about 9.2 nm and 10.5 nm for MPA-ZnS QDs and MPA-ZnS QDs respectively.

The crystallinity and the dimensions of the X-ZnS QDs were exhibited by X-ray diffraction (XRD). Fig. 3 shows the XRD pattern of the synthesized X-ZnS QDs. Three major diffraction peaks are located at 2θ values of ∼28 °, ∼47 ° and ∼56 ° corresponding to the lattice planes (111), (220), and (311) of cubic zinc-blende structure, which is consistent with the standard card (JCPDS No. 019-0191) and literature reports [START_REF] Ch | Influence of capping agent on structural, optical and photocatalytic properties of ZnS nanocrystals[END_REF]. In all the XRD patterns, broadening of the peaks indicates the nanocrystalline nature of the synthesized nanoparticles [START_REF] Liang | The role of the liquid-liquid interface in the synthesis of nonequilibrium crystalline Wurtzite ZnS at room temperature[END_REF][START_REF] Mansur | Beyond biocompatibility: an approach for the synthesis of ZnS quantum dot-chitosan nano-immunoconjugates for cancer diagnosis[END_REF]. In addition, no reflections related to impurities were detected in the pattern, indicating the high purity of the final product. The mean crystallite sizes, D, were estimated by using the Debye-Scherrer equation (Eq. 1):

𝐷 = 𝑘𝜆 𝛽 𝑐𝑜𝑠 𝜃 Eq. 1
where k = 0.9 is the shape factor, λ is the X-ray wavelength which is 1.54 Å, β is the full width at half maximum of the XRD diffraction peak in radian and θ is the Bragg diffraction angle.

The estimated crystallite sizes of the nanoparticles are 9.1 nm and 10.2 nm for MPA-ZnS QDs and MPS-ZnS QDs respectively, which are in agreement with those estimated from TEM images.

IR absorption

FT-IR analysis of X-ZnS QDs was conducted to check the bonding of ligands onto the ZnS quantum dots surface. Fig. 4 shows the FT-IR spectra of MPA-ZnS QDs and MPS-ZnS QDs.

The FT-IR spectra of MPA-ZnS QDs (Fig. 4a) show absorption bands at around 3287 cm -1

(𝜈(O-H)), 2914 cm -1 (𝜈 (CH2)), 1552 cm -1 (asymmetric 𝜈as (COO -)), 1395 cm -1 (symmetric 𝜈s (COO -)), and 665 cm -1 (𝛿 (C-S)), which are the characteristic peaks of MPA ligand [START_REF] Chang | γ-Radiation enhanced luminescence of thiolcapped quantum dots in aqueous solution[END_REF][START_REF] Ren | Aqueous synthesis of L-cysteine and mercaptopropionic acid co-capped ZnS quantum dots with dual emissions[END_REF]. The FT-IR spectra of MPS-ZnS QDs (Fig. 4b) show absorption bands at around 2912 cm -1

(𝜈(-CH-)), 1177 cm -1 and 1344 cm -1 (asymmetric 𝜈as (-S=O)), 1049 cm -1 (symmetric 𝜈s (-S=O)) and 729 cm -1 (νs (-S-O)), which are consistent with the presence of the sulfonate groups on MPS functionalized ZnS nanoparticles [START_REF] Cerra | Study of the interaction mechanism between hydrophilic thiol capped gold nanoparticles and melamine in aqueous medium[END_REF][START_REF] Venditti | Nanocomposite based on functionalized gold nanoparticles and sulfonated poly(ether ether ketone) membranes: Synthesis and characterization[END_REF][START_REF] Zhang | Colorimetric detection of paraquat in aqueous and fruit juice samples based on functionalized gold nanoparticles[END_REF]. As previously reported, the S-H stretching bond of thiol ligands was found between 2666 cm -1 and 2572 cm -1 in the spectra of the pure ligands [START_REF] Wang | Fluorescence enhancement detection of uric acid based on water-soluble 3-mercaptopropionic acid-capped core/shell ZnS:Cu/ZnS[END_REF]. As shown in Fig. 4, this band disappeared in the case of the MPA-ZnS QDs and MPS-ZnS QDs, demonstrating the attachment of the MPA and MPS ligands through covalent bonds between thiols and surface Zn atoms of ZnS QDs [START_REF] Do | Synthesis, optical characterization, and size distribution determination by curve resolution methods of watersoluble CdSe quantum dots[END_REF].

UV-Vis Absorption and Photoluminescence Spectroscopy

The absorbance and PL spectra of the X-capped ZnS QDs were studied to analyze the optical properties as shown in Fig. 5. As shown in Fig. 5A and5B, the absorbance spectra of X-capped ZnS QDs have broad absorption bands in the UV region which extends to the visible region with absorption edges at 272 nm and 295 nm for MPA-ZnS QDs and MPS-ZnS QDs respectively. The absorption edges are blue shifted in comparison with that of the bulk ZnS (334 nm) [START_REF] Nguyen | Investigations on photoluminescence enhancement of poly(vinyl alcohol)-encapsulated Mn-doped ZnS quantum dots[END_REF]. These shifts of the absorption wavelength are due to the size effect of the ZnS nanoparticles, which is consistent with the previous results [START_REF] Nguyen | Investigations on photoluminescence enhancement of poly(vinyl alcohol)-encapsulated Mn-doped ZnS quantum dots[END_REF].

The band gap energy of the X-ZnS nanoparticles can be evaluated from the UV-Vis spectra by using the relationship

εhν = A(hν -Eg) n
Eq. 2

where, α is the absorption coefficient, hυ is the incident photon energy, A is a constant and Eg is the band gap energy of the material. The exponent n depends on the type of the transition.

Here, the transitions are direct so we take n = 1/2. As shown in Fig. 5A1 It is observed that the optical band gaps are larger than that of bulk ZnS (3.6 eV) due to quantum confinement effect [START_REF] Nguyen | Energy transfer in poly(vinyl alcohol)-encapsulated Mn-doped ZnS quantum dots[END_REF][START_REF] Prasanth | Nonlinear optical property and fluorescence quenching behavior of PVP capped ZnS nanoparticles co-doped with Mn 2+ and Sm 3+[END_REF][START_REF] Juine | Surfactant-free green synthesis of ZnS QDs with active surface defects for selective nanomolar oxalic acid colorimetric sensors at room temperature[END_REF]. The Eg of the X-capped ZnS QDs has been calculated in order to estimate their particle size by using the Brus equation [START_REF] Liang | The role of the liquid-liquid interface in the synthesis of nonequilibrium crystalline Wurtzite ZnS at room temperature[END_REF] (Eq. 3)

𝐸 𝑔 𝑁𝐶 = 𝐸 𝑔 𝑏𝑢𝑙𝑘 + ℏ²𝜋² 2𝑟² ( 1 𝑚 𝑒 + 1 𝑚 ℎ ) - 1.8 𝑒 2 4𝜋ε 𝑟 𝜀 0 𝑟 Eq. 3
where 𝐸 g bulk is the bulk band gap, 𝐸 g NC is the band gap of nanocrystalline, ℏ is the Planck constant, r is the particle radius, me is the effective mass of an electron, mh is the effective mass of a hole, e is the charge of an electron, εr is the relative dielectric constant of the semiconductor, and ε0 is the dielectric constant of the vacuum. The obtained values of average diameter of the X-ZnS QDs (d = 2r) using the Brus equation are listed in Table 1 and are compared with that observed from XRD and TEM measurements respectively. The estimated particle size values of X-capped ZnS QDs are in agreement with that obtained from TEM and XRD studies.

Table 1.

Comparison of crystallite size of synthesized X-ZnS QDs estimated from different studies along with the calculated values of direct band gap energy.

The PL spectra of the MPA-ZnS QDs and MPS-ZnS QDs colloidal aqueous solutions upon excitation at 300 nm present symmetric peaks at ~438 nm and ~442 nm respectively with similar and narrow FWHM ~ 107 nm (Full Width at Half Maximum) (Fig. 5A and5B). Such features are the consequences of homogeneous size distribution and uniform crystallinity, which was confirmed by TEM [START_REF] Liu | Determination of rifampicin based on fluorescence quenching of GSH capped CdTe/ZnS QDs[END_REF][START_REF] Tedsana | A highly selective turn-on ATP fluorescence sensor based on unmodified cysteamine capped CdS quantum dots[END_REF].

The fluorescence quantum yield ΦQD was obtained using the protocol proposed by Würth et al. [START_REF] Mrad | Tailoring the photophysical properties and excitonic radiative decay of soluble CdSe quantum dots by controlling the ratio of capping thiol ligand[END_REF] according to Eq. 4 using fluorescein in water as a standard of quantum yield ΦFr = 0.89.

The UV absorption and fluorescence spectra of fluorescein and X-ZnS QDs in water were recorded successively under exactly the same conditions.

ΦQD = ΦFr

Optimal conditions for the enhanced fluorescence detection of TC by using X-ZnS QDs

Before the detection of TCs, some experimental parameters, including the pH effect and the concentration of X-ZnS QDs were carefully investigated.

With the objective to corroborate the surface charge on the QDs and the electrostatic interactions between the X-capped ZnS QDs and the TC, zeta potential was determined by varying the pH of the MPS-ZnS QDs (Fig. 6A). Both the zeta potentials of MPS-ZnS and MPA-ZnS QDs were negative within the pH range from 1 to 13, which is the result of carboxylate or sulfonate groups on their surfaces as recently reported [START_REF] Ramírez-Herrera | NIR-emitting alloyed CdTeSe QDs and organic dye assemblies: A nontoxic, stable, and efficient FRET system[END_REF]. The favorable formation of the complex X-ZnS QDs/TC requires that the TC is positively charged or neutral. negatively charged TC undergoes repulsive electrostatic interactions with ZnS-QDs. As shown in Fig. 6B, TC is positively charged at pH below 4 and it is neutral in the pH range from 4 to 7 [START_REF] Rivera-Utrilla | Tetracycline removal from water by adsorption/bioadsorption on activated carbons and sludgederived adsorbents[END_REF].

However, strongly acidic media are not recommended as the X-ZnS QDs can be dissociated and the interaction between X ligands and ZnS QDs can be wrecked, leading to a decrease in the stability of QDs [START_REF] Duan | Facile synthesis of N-acetyl-l-cysteine capped ZnS quantum dots as an eco-friendly fluorescence sensor for Hg 2+[END_REF][START_REF] Cao | Facile synthesis of chitosancapped ZnS quantum dots as an eco-friendly fluorescence sensor for rapid determination of bisphenol A in water and plastic samples[END_REF][START_REF] Mandani | Development of a new simple spectroscopic determination coupled acid-motivated delivery system based on fluorescence turn-off MSNs@MPA-ZnS QDs for infection[END_REF]. Therefore, pH = 5.6 has been chosen for the formation of the X-ZnS QDs/TC complex through hydrogen bond and the detection of TC by fluorescence quenching. respect to concentration is linear up to 0.2 mg•mL -1 ; it levels off at higher concentrations (Fig. 7). The highest fluorescence while keeping in the linear domain is 0.2 mg•mL -1 . This concentration was selected as the optimum X-ZnS QDs concentration because fluorescence quenching by TC decreases the fluorescence intensity, thereby keeping the fluorescence signal in the linear domain. The calibration curve of MPA-ZnS QDs presents a good linearity within the total concentration range of TC with a regression coefficient R 2 = 0.9994. this linearity is possibly due to static quenching mechanism [START_REF] Seedher | Fluorescence spectroscopic study for competitive binding of antidiabetic drugs and endogenous substances on serum albumin, Drug Metabol[END_REF]. In contrast, the calibration curve of MPS-ZnS QDs presents a downward curvature plot (Inset Fig. 8C) at low concentration with a good linearity only within the concentration range from 200 nM to 6000 nM (R 2 = 0.9977). The downward curvature plot at low concentration is possibly due to the dynamic quenching with a static quenching mechanism for the rest of the concentration range [START_REF] Santra | Charge-transfer-induced fluorescence quenching of anthracene derivatives and selective detection of picric acid[END_REF][START_REF] Mehata | An efficient excited-state proton transfer fluorescence quenching based probe (7hydroxyquinoline) for sensing trivalent cations in aqueous environment[END_REF]. The calculated Stern-Volmer quenching constants KSV (which represents the quenching efficiency) from the linearized equations Eq. 6 The merits and the detection limits of X-ZnS QDs probes for measuring TC are briefly summarized in Table 2. It can be seen that MPA-ZnS QDs present lower LOD and larger linear range compared to that of MPS-ZnS QDs and therefore better sensitivity. This result shows that the type of ligand functionalized on ZnS QDs can affect the performance of the sensor. To better explain the obtained result, we have based ourselves on hypothesis linked with the length of the ligand carbon chain. Indeed, the MSA ligand has a longer carbon chain than the MPA ligand, which increases the probability of the MSA ligand's carbon chain deformation. Consequently, fewer hydrogen bond will be resulted (Fig. 9). Unlike MPA ligand can lead to an orderly and more stable distribution on the surface of ZnS QDs which can increase the hydrogen bond interaction between MPA and TC (Fig. 9). The analytical performance from point of view limit of detection and linear range of the MPA-ZnS QDs for TC detection was compared to previous probes (Table 3). It can be seen that the MPA-ZnS QDs probe reached a very low limit of detection in the order of picomolar that was not achieved before. The lower LOD and large linear range obtained in this work demonstrate the higher sensitivity of our proposed MPA-ZnS QDs towards TC detection compared to other reported assays (Table 3). In addition, this result proves the higher sensitivity and efficiency of the fluorescence method that possessed a simple and fast detection procedure without requiring a special reaction medium or complicated preparation processes as required by the conventional methods [START_REF] Gu | Ultrasensitive fluorescent detection of tetracycline based on selective supramolecular interaction of nitrogen chlorine co-doped graphene quantum Dots[END_REF]. It should be noticed that the relative standard deviation (RSD) for three replicate detections using MPA-ZnS QDs was 3.85 %, indicating that this nanoprobe has appreciable reproducibility. Based on the lowest detection limit and wide detection range, the above analysis results prove that the MPA-ZnS QDs can be employed as an acute candidate for the detection of TC, yet proper selectivity tests should then be imposed to understand the specific accuracy in the measurement.

Table 3. The comparison of MPA-ZnS QDs with other reported probes of TC fluorescence detection in recent years.

Probes

Linear range (μM)

LOD (µM) references

MIPs-AA/CQDs molecularly imprinted fluorescence sensor with carbon quantum dots and acrylic acid 1.0-60 0.17 [START_REF] Wei | Preparation of molecularly imprinted fluorescence sensor based on carbon quantum dots via precipitation polymerization for fluorescence detection of tetracycline[END_REF] (TAP)/ γ-cyclodextrin triple-helix aptamer probe/cyclodextrin 0.005-0.1 0.0016 [START_REF] He | A sensitive fluorescent assay for tetracycline detection based on triple-helix aptamer probe and cyclodextrin supramolecular inclusion[END_REF] Silver nanoparticle-encapsulated halloysite nanolumen 0.05-10 0.0124 [START_REF] Xu | Metal-enhanced fluorescence detection and degradation of tetracycline by silver nanoparticle-encapsulated halloysite nano-lumen[END_REF] Silicon quantum dots 0.0113-1.086 0.0009 [START_REF] Liu | A novel fluorescence probe for rapid and sensitive detection of tetracyclines residues based on silicon quantum dots[END_REF] Aptamer-labelled fluorescent nanoprobeintegrated shell isolated nanoparticle-enhanced fluorescence spectroscopy 0.2-400 0.0162 [START_REF] Wang | Silica-coated silver nanoparticles decorated with fluorescent CdTe quantum dots and DNA aptamers for detection of tetracycline[END_REF] 3-Mercaptopropionic acid-ZnS quantum dots 0.025-6 0.00003 This work

Conclusion

The present investigation provided a simple, low cost, efficient and eco-friendly fluorescence sensor for direct detection of trace TC with high affinity using health-friendly ZnS QDs as a fluorescent probe. The ZnS QDs was firstly capped with two different and well-known MPA and MPS ligands as stabilizers. The successful formation of X-ZnS QDs was confirmed by FTIR characterization. The obtained X-ZnS QDs nanoparticles with a size of 9-10 nm have good colloidal stability, uniform sizes, good cubic zinc blended structures, strong quantum confinement effect and high fluorescence properties. Taking into account favorable conditions for the electrostatic interaction between target analyte and nanoparticles, X-ZnS QDs were applied for the direct detection of TC based on the quenching fluorescence of nanoparticles. The results have shown that MPA-ZnS QDs presents better analytical performance than MPS-ZnS QDs.

MPA-ZnS QDs have demonstrated its ability to detect traces of TC with LOD of 30 pM, large linear range from 200 nM to 6000 nM and good reproducibility of the measurements. Under optimal conditions, MPA-ZnS QDs has shown better analytical performance towards the detection of TC compared to that MPS-ZnS QDs. The simple and low cost MPA-ZnS QDs fluorescence sensor has achieved a very low limit of detection at picomolar level which is rarely reached compared to recently reported fluorescent sensors. According to such high analytical performance, we believe that the application of MPA-ZnS QDs in analysis of food products has great potential.

Fig 2 .Fig. 3 .

 23 Fig 2. TEM images of (A) MPA-ZnS QDs and (B) MPS-ZnS QDs.

Fig. 4 .

 4 Fig. 4. FTIR spectrum of (a) MPA-ZnS QDs and (b) MPS-ZnS QDs.

Fig. 5 .

 5 Fig. 5. Absorption (blue curves) and Photoluminescence spectra (red curves) of (A) MPA-ZnS QDs and (B) MPS-ZnS QDs. Tauc's plot for the determination of the optical band gap of (A1) MPA-ZnS QDs and (B1) MPS-ZnS QDs.

FQDs

  and FFr are the spectrally integrated photon fluxes at the detector for both samples, i.e., the area under the emission spectrum. I Abs is the fraction of the excitation light absorbed by the chromophore or sample. F i (λex, λem) is the instrument function comprising both the spectral profile of the excitation and the response of the optics and the detector of the fluorescence spectrometer, within the spectral range of the samples. Since we have used the same excitation wavelength and exactly the same settings and parameters for both samples, the instrument function is the same for both samples.The quantum yield of the MPA-ZnS QDs and MPS-ZnS QDs have been calculated to be ΦQD = 11 % and 10 % respectively, which makes the X-ZnS QDs suitable to be used for biological fluorescent labeling and carrier for guest materials

Fig. 6 .

 6 Fig. 6. (A) Zeta potential of the MPS-ZnS QDs at different pH values, (B) pH-dependent speciation of tetracycline molecule.

Fig. 7 . 3 . 5 . 5 F0

 7355 Fig. 7. (A) The fluorescence intensity of MPS-ZnS QDS solutions under different concentrations, (B) The fluorescent intensity of MPS-ZnS QDs versus its different concentrations.

  3.27 10 -4 [𝑇𝐶] ; (R 2 = 0.9977); [200 nM to 6000 nM] Eq. 7

and 7

 7 were found to be KSV = 9.20 × 10 5 L•mol -1 and KSV = 3.27 × 10 5 L•mol -1 for MPA-ZnS QDs and MPS-ZnS QDs respectively. The high values of Stern-Volmer quenching constant KSV for both MPA-ZnS QDs and MPS-ZnS QDs demonstrate excellent quenching efficiency as well as good affinity towards TC. On the basis of KSV values and the standard deviation of the blank measurement (δ), the corresponding detection limit (LOD) was determined based on IUPAC criteria, 3δ/KSV [54].

Fig. 8 .

 8 Fig. 8. Fluorescence spectra of (A) MPA-ZnS QDs (0.2 mg/mL) and (B) MPS-ZnS QDs (0.2 mg/mL) upon the addition of various concentrations of TC (from top to bottom, 0, 25, 50, 100, 200, 400, 1000, 2000, 3000 and 6000 nM). (C) Linear fitted result using Stern-Volmer plot of TC concentration dependence on fluorescence intensity of MPA-ZnS QDs and MPS-ZnS QDs. Spectra were recorded with excitation at 300 nm.

Fig. 9 .

 9 Fig. 9. Schematic representation of the electrostatic interaction between the positive charges of TC and the negative charges of (A) MPA-ZnS QDs and (B) MPS-ZnS QDs, highlighting the effect of ligands length and their distribution on the interaction with the target analyte.

Table 2 .

 2 The comparison of the obtained analytical performances of MPA-ZnS QDs and MPS-ZnS QDs.

	Probe	Linear range (nM)	Limit of detection (nM)
	MPA-ZnS QDs	25-6000	0.03
	MPS-ZnS QDs	200-6000	0.1


Hunting and livestock farming in early La Tène in Alsace, the example of Obernai PAEI

The bones remains

The early La Tène structures excavated at Obernai (450-375 BC) outline an important rural settlement with a two hubs organization. They delivered a corpus of bone and dental animals remains composed of 1569 remains with a total mass of approximately 38 kg. This significant mass is due to the presence of thirteen animal skeletons, domestic and wild, more or less complete deposited in silos.

The other remains constitute a characteristic rejection of meat consumption. Cattle, pig and caprineas are mostly consumed. Dog and equidea are also consumed but occupy a secondary place. Some wildlife remains, deer and boar, are also present in this rejection and show anthropogenic traces which confirm the consumption of theses species

Domestic animals deposit

These deposits are composed of dogs, sheeps and pigs. None of this subjects revealed any anthropogenic trace or significant absence of somes anatomical parts related with their consumption. Their position in the structures indicates that they were carefully deposited; thus, it does not appear to be a simple rejection of carcass. All dogs were found deposited in association with sheeps. The osteometry indicates that the dogs haven't the same morphology as those bearing consumption traces. These dogs don't seem to be for slaughter. The presence of caprinea and dogs deposited in domestic context is a common phenomenon for the period. It's difficult to found a explanation to this association between dogs ans sheep but it is not unreasonable to suppose that these deposits are the reflexion of pastoral activites.

Deers deposits

Three deer were deposited in silos, two males and one female. None of these subjects, althaugh they are carefully deposited, aren't complete. One in particular could be a trophy. Despite the absence of somes anatomical parts, who reflect a partial consumption, all were discovered in connection. These bones don't bear antropogenics traces in relation to meat consumption. These deposits reveal a cynegetic activities in connection with aristocracy.

Discussion

If the animal deposition phenomenon in silos is very common in Alsace and in France during the early La Tène, the deer presence in these particular contexts makes of Obernai a case that we haven't encountered anywhere in literature. Their presence and all the staging around it reveal the importance of the hunting for this population. This is also visible in the consumption rejection where wild species are presente in a proportion of 5%. At the same period, at Geispolsheim, their presence don't exceed 1%, with no remain of big game except deer antlers. At La Tène final period (130/60 BC) at Obernai, their portion is small (3,4%).

These comparisons show a differentiated use of natural environment according to the periods of Iron Age and as well as the sites. At Geispolsheim, an occupation richer than Obernai, no staging highlights the role of hunting, which is barely represented.

The Obernai's deposits demonstrated the importance of breeding and hunting for the population of this site. The first is more common for this period, but the second seems to play an unusual role.

3

  Photo : É. Guttman