Michael Tsangaris

Keywords: recuperation, punk scene, graffiti, tattoo, social media, Situationist International

Re-adapting radical forms of expression in the digital era -Investigating new aspects of recuperation 1 .

Introduction

According [START_REF] Becker | Outsider[END_REF][START_REF] Becker | Art Worlds[END_REF], art and deviance are settled by social conventions that usually follow some typical processes of social interaction. Individuals and social groups act according to sociallydefined labels that are established and maintained over time as protocols. Besides, the repetition of some typical actions affirms them as objective entities and establishes collective conventions that define what reality is [START_REF] Berger | The social construction of reality: A treatise in the sociology of knowledge[END_REF]. As all these conventions and patterns are established and institutionalized, a shared sense of reality is constructed, confirmed and reproduced [START_REF] Searle | The construction of social reality[END_REF].

As early as the nineteen-sixties, the Situationist International (SI) declared that the late capitalist society that was based on mass commodity production had already caused a significant qualitative change in people's experience of everyday life, which led initially to the downgrading of being into having, and then from having to appearing [START_REF] Debord | Society of the Spectacle[END_REF].

The concept of the spectacle does not refer only to simple theories of manipulation by using labels that will progressively lead to social conventions, it defines decisively what social reality is. The spectacle is much more than an institution of socialization, or a propaganda mechanism that merely seeks to influence opinions. The spectacle has its roots in representation that is considered an essential human quality.

Humans understand and communicate their lives deductively as narratives [START_REF] Bruner | Life as narrative[END_REF]). Human experience is inherently multisensory. Each human sense provides different frameworks of experience. Expressions very often use representation as a tool for communication. Representation that is the basis of the symbolic culture, presupposes a separation between the form and the concept, thus the symbolic expression and the actual thought. This separation imposes a mediation that always changes the original meaning. Thus, signs and symbols are always mediums that restrict the communication of the immediate human experience through certain channels, most regularly ideologically orientated, by abstraction, presenting selective perspectives that misrepresent the totality of the social process. Symbolic culture filters perception via the interpretation of social reality through the signs. [START_REF] Zerzan | Mirrorpix in Walters S, 'Music & Nightlife Manchester Music Four Manchester bands we owe to the Sex Pistols[END_REF] puts under consideration the axiom that symbolic thought is an intrinsic human quality. Perhaps symbolic representation has developed as a false communication experience, either a cultural or psychological defection. In any case, the symbolic way of abstract expression and comprehension can limit communication, and the excessive dependence on this process leads to objectification, alienation and preconception. According [START_REF] Goody | Representations and Contradictions Ambivalence Towards Images, Theatre, Fiction, Relics, and Sexuality[END_REF] for many generations before 'civilization', people did many things with their minds, including communication, without the symbolic process that is primarily a closed system. However, when Debord talked about the 'Society of the Spectacle', he was mostly referring to the aggravation of this phenomenon in late capitalism and considered that the excess begun sometime in the third decade of the twentieth century (Debord 1998: 4). According to Jappe (1991: 21), the spectacle at this excess appears in many guises: different political views and tempers, contrasting life styles, antagonistic artistic attitudes and so on. People are urged to express their opinion and to choose one of the false alternatives, so that they will never question the whole.

A theoretical summary of the concept delineates spectacle as the 'subjugation of the world to the economy, the fetishism of goods, reification, alienation, ideology, and specifically, how images, representations, entertainment prevent authentic life from coming into being' (Kaufmann 2003: 160).

Actually, the concept of the spectacle defies precise definition because it is usually employed as a concrete proverb to convey all the negative parts of the late capitalism.

In sum, the spectacle for the SI embodied the separation between symbolic expression and actual human activity and thought, and all the consequences that this fact brought, such as the separation of power, the division of labour, the market economy, the omnipotence of the image and the extension of capitalist relations to every aspect of life. Thus, the concept of the spectacle is used as a theoretical model to describe how power functions from several apparently unrelated, unprejudiced sources to achieve a standardized effect for late capitalism (Crary 1989).

According to the SI capitalism is not a creative force, but a recuperative one. In the case of labor for example, human creativity and ingenuity are being recuperated to mass production that alienates the workers and, in this respect, the real social value of creativity is objectified and reduced to routine and work to be done. In the society of the spectacle the human creativity and the human expression are used not to serve for personality fulfillment but to make profit and reproduce the same system (Jappe 1999: 99).

As the capitalist system is constantly produced and reproduced at an ever-higher level, the form of consolidation becomes more and more profound with reflective consequences on human consciousness until it becomes a component of it (Lukács 1923: 9). According [START_REF] Duncum | Theorizing everyday aesthetic experience with contemporary visual culture[END_REF] ideology deals with ordinary cultural artefacts and it can be hard to resist because it regularly appears to belong to the realm of the natural. Visual pedagogy can uncover ideology within the realm of the everyday life, exploring what lies behind images and more specifically the material conditions of their production, distribution and the specific interests that they serve (Duncum 2001), preparing in this way the students for resistance against oppression by hidden forms of power.

Capitalism has adopted recuperation that is an armor that the spectacle provides to prevent damages being inflicted by apostates of the system. The spectacle embodies a property that mechanically protects it, that is, the selective nature of representation that makes possible the process of recuperation. Representation, by selecting only harmless parts of the deviance and the unwanted, shapes forms compatible to the system, assimilating political, social and cultural opposition to capitalism through the spectacle. In this respect, contrasting concepts can be compiled and unified into commercial forms. In the society of the spectacle everything becomes possible, that is commercial rebellious music, street art galleries, fashionable counter-culture emblems, etc. and follow up absurd hybrid products such as the Che Guevara vodka, the Tattooed Barbie, etc.

According Knabb (1981: 372), the SI used the term recuperation in the sense that the system is recovering something that has escaped and revolted, avoiding in this way a potential overthrown. The recuperation of a radical act or idea can be realized by separating parts from the total revolutionary claim and incorporating them into the spectacle as cultural elements, while not necessarily obtaining the approval or implementation by the part of the system or the resistance. Co-option (co-optation) is a similar concept that was coined by Selznick in his study 'Foundations of the Theory of Organization' (1948). However, co-option seems to obtain the approval or implementation by the system and the opposition 2 .

In his mythologies [START_REF] Barthes | Mythologies[END_REF], gave examples of this process. A small 'inoculation' of an 'acknowledged evil' can protect the bourgeois establishment against the risk of a generalized subversion. The signs through which the capitalist society projected the subversive elements needed in the consumer culture bear very little relation to their actual strength. Isolated from the real context, these signs of subversion can become symbols that may produce different effects. They can exploit consumers eagerness to identify with rebellious and anti-authority potentialities and present their indignation for social injustice in their daily shopping. Thus, the business world exploits the images and introduces a second order of signification, a secondary myth that symbolises something different from the initial meaning. Rebellion is accessible through consumption. The rebellious mythical image is appropriated to sell all sorts of products and services, (t-shirts, beer, financial services, etc.). When people buy products with counter-cultural emblems on them, they think they are making personal statements of rebellion and anti-capitalism. Nevertheless, those emblems have become part of the commodity system and so the acquisition of those products support consumer society. Commercial culture takes a real concept that may have dangerous implications for the system and turns it into an image which as Barthes pointed out, has been safety re-invented as a myth.

Still, as Combre (2011) has investigated the possibility that both processes of commodification and radicalization of Che Guevara's image can coexist. In her work reviewed that there is no real evidence that revolutionary images can be de-politicized by commercialism after all, and in this sense they may continue to function as a virtual prosthetics for political ideas.

In any case, the visual investigation of former counter cultural emblems and their adjustments through time, can be a prolific pedagogical tool for fostering students with social awareness, enabling them to realise some of the methods that capitalism uses for conditioning radical reactionaries and to comprehend the 'political passivity bred by the culture of illusion' [START_REF] Hedges | Empire of illusion: The end of literacy and the triumph of spectacle[END_REF].

Scholars that studied the various youth (and adult) cultures within societies used the terms countermores [START_REF] Lasswell | World Politics and Personal Insecurity[END_REF], contra-cultures [START_REF] Yinger | Contraculture and Subculture[END_REF], counter-cultures [START_REF] Roszak | The making of a counter culture: reflections on the technocratic society and its youthful opposition[END_REF], sub-cultures (Hall and Jefferson 1993) and most recently, the flexible shapes of post-subcultures, neo-tribes [START_REF] Brookman | Forever Young: Consumption and Evolving Neo-Tribes in the Sydney Rave Scene[END_REF] and scenes [START_REF] Bennett | Music scenes: Local, translocal and virtual[END_REF][START_REF] Straw | Some things a scene might be: Postface[END_REF][START_REF] Tsangaris | Frame Analysis and the Rock Scenes[END_REF]. All those scholars investigated from various perspectives those cultures, as youth socialization mechanisms, as social class homologies, as deviance forces, as social movements, as marginalization or integration systems, etc. always in relation to the dominant culture 3 . However, anti-establishmentarianism always exists inside these social groups, embodied usually to their extreme and radical core. The values and forms of all those categories necessarily differ and conflict in some measure with dominant culture. Personality plays also a significant role in this argument, so individuals that follow these categorizations possess rates of rebellion, in respect to their personal criteria and their integration to each group or community. In this sense, contemporary counter-cultures, 3 According Clarke [START_REF] Clarke | Subcultures, cultures and class[END_REF] 'sub-cultures' that have working class origin and clearly articulated collective structures close to gangs are fluctuating between the constraints of dominant institutions (home, school, work) and those of leisure (peer-group associations, etc.). Most possibly are socially introverted, representing an appropriation of the 'ghetto'. 'Counter-cultures' have middle-class origins, are less group-centered and more individualized. Most possibly they are socially extroverted but tend to construct enclaves within the interstices of the dominant culture, struggling for an 'alternative society'. In this article, without considering class homologies, I use the term counter culture as any social group that is opposed to the dominant culture. While counter cultures are always against the dominant culture, I consider the term sub cultures wider, including also cultures that not necessarily operate always against the system. just like the old delinquent gangs investigated in the fifties by [START_REF] Yablonsky | The Delinquent Gang as a Near-Group[END_REF], are characterised by an imprecise definition of boundaries.

The separate values and norms of counter cultures may involve many aspects of life such as, systems of communication (language, dialects, art, fashions etc.), systems of beliefs (ideologies, ethics, ideals etc.) and social organization (group parameters, types of administration, policies and strategies etc) and social practices, (demographics, laws, behaviour, diets, customs, etc.). All these norms are learned by interaction within the counter culture community.

A basic activity for the existence of counter cultures that belongs to the symbolic systems of signification is the creation and the maintenance of distinct visual characteristics and emblems. The visual identities of counter cultural groups although presuppose social labelling, stereotyping and stigmatisation, they provide also internal coherence and distinction from conventional appearance and the mainstream fashion.

Although there has been a great variety of symbols and practises (visual, rituals, activism, etc.) with which the counter cultural scenes have been periodically expressed, at this point I will list some of the most common such as black clothing, tattoos, spiky or shaved hair, mohawk, ripped jeans, bovver boots, the circle-A, the black flag, the peace sign, the swastika, the skull and crossbones symbol, wall slogans, stencils, graffiti, motorcycles, black leather jackets, body piercing, etc.

On this this study I will not focus on the analysis and the differences of diverse cultural groups, but instead I will review how counter cultural visual emblems and practises are defined, categorized, redefined and recuperated by the dominant culture.

Basis of the approach

This study investigates contemporary developments of visual signification concerning former emblems of resistance in the contemporary mixed media environment. It is a part of a larger theoretical project that examines historically the social, ideological and aesthetic dimension of recuperation of counter cultures from the dominant culture.

The examination of the circulation and evolution of alternative visual elements in contemporary society, and more specifically the investigation of counter-cultural emblems interaction with the mainstream culture is very important for a better understanding of the recuperation process.

Though for the past decades counter cultural emblems belonged to the sphere of alternative media and followed narrow diffusion, nowadays the digital communication technologies and the Web 2.0 applications have enhanced the capacities of their circulation. This article should be primarily understood as a theoretical and explorative review for the concept of recuperation, although it is supported largely by visual material selected mostly by the social media, to highlight the changes that the new communications systems brought. In a world almost totally mediated by the spectacle, supported from a variety of media, counter cultural emblems after repetitive appropriation become banalities. Without retrospective inspection and visual evidence from the past, it is difficult for someone to realise how visual representations are being constantly encoded and reencoded, changing cultural meanings over time.

As [START_REF] Tavin | Wrestling with angels, searching for ghosts: Toward a critical pedagogy of visual culture[END_REF] notes critical pedagogy 'supports the study of popular culture in order to understand and challenge the way subjectivities are constituted through images and imagining'. Accordingly, critical visual pedagogy concerning the history and the evolution of counter cultural emblems, apart the fact that enhances human skills in pictorial perception and understanding is also valuable for the recognition of the way cultural appropriations and meaning adjustments are constituted through images. In this sense it can empower students to notice and meaningfully involve in the contemporary ideological and cultural struggles.

The data of this study are selected from different sources such as social media, blogs and alternative counter-culture web sites, advertisements, illustrated books and albums.

Past and contemporary images are collocated in couples, according their emblem thematic, in order to present the contrasting result of their visual confrontation. Beneath each pair of images follows a brief semiotic discourse that simplifies decoding, with reference to the dominant culture of the time they were produced respectively.

In this sense, based upon real-world contemporary media practices and representations, by bringing up historical and theoretical references, I will present an instructive framework concerning recuperation and the complexity of the present-day visual culture.

Applications of visual recuperation

In order to explore visually how does the process of recuperation operate, I will present particular alternative communication modes that have been used by social groups formerly for the symbolic construction of resistance against the dominant culture, referring also to their contemporary meaning. In this respect, visual symbols and ritual practices such as punk haircuts and clothing, tattoos, wall slogans, graffiti and stencils, although once served to express and to conceptualize the differentiation and the antithesis between subversive groups and the establishment, today are re-organized in the terms of the main culture and provide a different meaning.

During the seventies mohawk hairstyles, dog collars and studded black leather jackets were typical accessories of the punk generation. Punk is one of the most prominent examples of countercultures that can be very convenient for the purposes of this analysis as it was mainly using symbolic communication practices. It was a radical youth rebellion that first emerged during the seventies, against the difficult political economic and social conditions of the recession that affected most of the Western World. It turned up also as an objection against the state of apathy that prevailed at that time concerning youth cultures and music scenes. Punk included a great number of distinctive styles of clothing and adornment (stripped jeans, offensive T-shirts, studded black leather jackets, shaved or spiked hair, bondage accessories etc.). The punk scene initially developed through alternative communication channels (bootleg recordings, gigs, stencils, fanzines, etc.), however, very soon a great part of it was also promoted by the mainstream media (big record companies, radio stations, the music press, big concerts etc.). It expressed a great number of rebellious viewpoints ranging across various political and social spectrums, from extreme anarcho-punk collectives and radical leftists to nazi punks, fascist skinheads, part-time teenage punks, hardcore, straight edge, and so on. Although their significant internal variations, the punk neo-tribes were all characterized by a tension for revolt and subversion challenging the contemporary culture. Forty years later, a web-site that presents the year's top ten haircuts published an image, depicting a gentle and moderate male wearing an expensive suit with a mohawk hairstyle. Obviously, the conventional signifying system and ethical codes that correlate signifiers and signifieds, concerning hairstyles and appearances have been reformed. Today, the mohawk haircut does not recall any more social marginality but a trendy fashionable style. Additionally, there is a text following in the webpage, giving the practical advice that in order to look 'catchy and handsome, you have to care about your appearance and it will be a perfect idea to change your look from time to time'.

In this manner, the image of the mohawk gradually lost its power as a punk emblem and the symbolic link that related this hairstyle to anti-authority worldviews was abolished once and for all. Ripped clothes for centuries were associated mostly with the less fortunate. However, during the seventies the photograph of four males with long mop-top hair, black leather jackets, short t-shirts and ripped straight jeans was one of the images that constructed the Ramones myth signifying the concept of the 'teenage rebel' (Picture 3).

A contemporary commercial web advertisement that promotes a pair of ripped jeans (Picture 4) inevitable imposes that 'the times they are a changin'. Certainly, popular fashion is a meaning system and a general trend influenced by several factors such as the cloth industry, the mass media, the music scenes, gender, class, ethnicity, etc. The continuing demand for profit keeps the fashion industry changing and as a result the trends can accept anything as a possible style line. Originally, ripped jeans, torn clothing and the safety pins were typical emblems of the Punk scene. Later in the nineties the Grunge scene adopted also similar emblems. After the millennium, the ripped jeans became gradually widespread by commercial fashion and still they are very common.

The Ramones myth has been gradually dismantled by the market, and its demystified parts gained clear commercial meanings. The cultural industry finally dissolved a typical image of the 'teenage rebel' by commercializing parts of it.

Picture 5. Sex Pistols in the seventies Picture 6. Virgin Money Debit Cards 2016

The Sex Pistols (Picture 3) activated their myth as basic emblems of the seventies counter-culture after releasing the song 'Anarchy in the UK' in 1976. They achieved anchoring the concept of counter-culture to their image, although in reality they can be considered as one of the punk bands that were co-opted very soon with the rules of the market 4 .

The year 2016 that was officially celebrated as the fortieth anniversary of Punk rock, an 'Anarchy in the UK' credit card was designed by a multinational corporation, so the consumers can 'put a little bit of rebellion in their pockets' [START_REF] Jones | Punk rock brand: the Sex Pistols to feature on a range of credit cards[END_REF]. The marketing managers realised that labelling a product as 'radical' is a very efficient technique to increase sales. It is based on the idea that gives the opportunity for the consumers to oppose and express their covert anger against the injustice of late capitalism.

Although the image of Sex Pistols can be still perceived as a connotation for the early punk music scene, it is relocated to a peculiar type of superficial commercialized rebellion, and in such a way it converts social discontent and subterranean values into mechanisms that motivate the consumer.

Tattoo culture in the Western World is quite old. Although there was a short trend for the social elite to get tattoos at the turn of nineteenth century [START_REF] Kosut | Tattoo Narratives: The intersection of the body, self-identity and society[END_REF], during the twentieth century the broader middle class considered tattoos as deviant practices that characterized specific social groups like sailors, mercenaries, soldiers of the lowest military rank, bikers, prisoners, drug addicts, punks, rockers, hooligans, gangsters, skinheads etc. Additionally, many people that were coming from the lower working class were attracted by the tattoo culture as well.

In this respect, people that had tattoos developed as a segregated heterogenous counterculture that carried signs of resistance against the white, heterosexual, middle-class cultural domination [START_REF] Demello | Bodies of inscription: A cultural history of the modern tattoo community[END_REF].

During the nineties tattoos became commercialized by the mainstream media through celebrities that wanted to express creatively themselves, and since then the popular acceptance of tattoos as an artistic personal emblem became very common. Today, tattooing is a common way for people to express aesthetically their own desired images but is also a phenomenon that reflects cultural influences [START_REF] Kosut | Tattoo Narratives: The intersection of the body, self-identity and society[END_REF].

The image of male young rocker with greasy hair and lots of tattoos (Picture 7) would generated 'moral panic' [START_REF] Cohen | Moral panics and folk devils[END_REF] to the bourgeois society of the sixties. Representations of youth subcultures at that time was connected to hooliganism, street fights, riots and generally public disturbance. Forty years after the connotations of these images have changed. The present-day marketing communication practices use concepts such as rebellion, distinctiveness and vanity as basic factors to influence consumer behavior.

Today, post-modern consumers can buy a variety of identities and attitudes embedded to products [START_REF] Evans | Consumer behaviour[END_REF], moving away from the model of the homogeneous mass consumer. A jeans advertising campaign in 2009, presented in a series pictures Josh Beech, a young British musician wearing a white tank top with his hands full of tattoos (Picture 8). Although both images presented moody tattooed males, the recent one has lost the original concept of deviation that characterizes the old one. Instead of generating a sense of 'moral panic' to the public, it suggests a cool identity for the young consumers. Skinheads emerged as a distinct subculture in Great Britain near the end of the sixties. They developed a way of look that included shaved hair, steel toe-capped polished boots, short rolled up jeans, green army nylon flyers jackets and tattoos. They adopt this outfit to articulate impressions of masculinity, 'hardness' and 'working-classness' in order to achieve uniformity and unity of their community [START_REF] Clarke | The skinheads and the magical recovery of community[END_REF]. They represented a counter-cultural movement that was against not just the middle-class prevalence, but aggressive to everyone that had pretensions to social superiority. Skinheads were also obsessed with football and during the eighties in the revival of the movement a great part of them had tendencies to nationalism, fascism racism and hostility toward minorities and immigrants [START_REF] Knight | Skinhead[END_REF]. Tattoos were one of the basic emblems of the skinhead culture during the eighties. An image of a tattooed skinhead at that time would certainly signify trouble, bringing up feelings related to fear and 'moral panic'.

The commercialization and popularization of tattoo after the millennium brought together a variety of comics, books and toys for the young consumers such as Tattoo Parlor Kit, tattooed Barbie, tattooed Lego, etc. The 'Lego Tattoo Campaign' for pens in 2010, mimicked the masculine and hard appearance of tattooed skinheads and bikers, shifting those associations to the product as values. As tattooing has been transformed from a legendary deviant code to a common cultural custom, advertisers can relate tattoos with commercial products.

Thus nowadays children are growing up, and playing with tattoo associated toys, in a society that is undoubtably tattoo-friendly than any other time in western history [START_REF] Kosut | An ironic fad: The commodification and consumption of tattoos[END_REF](Kosut : 1036)). The motorcycle is almost a sacred object for biker gangs because it has the special power to transport them and to offer them experiences beyond the ordinary. Harley Davidson is one of the favourite bikes for outlaw motorcycle clubs. Bikers customize their motorcycles according their own tastes, changing the colour, the fenders, the handle bars, the seats, the gas tanks, etc. Sometimes, they even rebuilt them mechanically adding extra power to the engine. In this sense, they feel released by the capitalist models of mass serial production and the Fordist uniformity and express their individuality. Their aim is to look bad and different to show the world that they are not part of conventional society [START_REF] Hopper | Hell on wheels: The outlaw motorcycle gangs[END_REF]. In honour of their special relation with their bikes, the motorcycle outlaws get decorated corporate motorcycle logo tattoos (picture 11). Far from advertising, the significations that those tattoos basically express is gang unity, freedom, disobedience and rebellion. In this occasion the tattooing of motorcycle brands functions as a détournement 5 and is clearly a ritual that belongs to the anti-cultural sphere.

The 'supermarket era' of tattooing [START_REF] Atkinson | Tattooed: The sociogenesis of a body art[END_REF] increased the popularity of corporate logo tattoo that carries signifiers of consumerist society reinforcing the process of the commodification (Orend and Gagne 2009). Posts on the social media advertise temporary corporate logo tattoos, suggesting that is 'a great way to make your next corporate event or grand opening unforgettable' (picture 12). In this sense, the omnipotence of consumerism supported by the social media demystifies counter-cultural rituals and disactivates détournements in order to recuperate them back to safe as practices of marketing. Wall slogans and graffiti for many years functioned as alternative media that can communicate social arguments and discontent by the oppressed social groups who are normally excluded from media production, contesting for the advancement of a co-operative society [START_REF] Downing | Radical Media[END_REF][START_REF] Fuchs | Alternative media as critical media[END_REF]. Very often alternative media are connected to community uprisings, protests and social movements that belong to the sphere of resistance [START_REF] Downing | Social movement theories and alternative media[END_REF]. In May of 1968, a great number wall slogan made by the SI were widespread all over Paris. One of their typical slogans that was graffitied since 1953 addressed to people 'never to work' (Picture 13). The slogan supported the struggle against workers alienation, representing counter cultural beliefs for the abolishment of capitalism.

In opposition, contemporary promotional graffiti on canvas, seems to represent the values that most alternative communication practices traditionally struggled to confront, that is, the prevalence of mainstream messages that are inclined by the consumerist culture. Picture 14 presents clearly the cooption of graffiti with the advertising film production.

Picture 13. Never Work Picture 14. Puma Graffiti on Canvas -Advertising production Advertisements assimilated radical media aesthetic forms and used the same techniques that the marginal groups used. Stencil is a technique for the visual reproduction of the same pattern many times, by applying colour through a layer surface with designed gaps. Unauthorised stencils can be traced in the late thirties during the Spanish Civil War and at the same time in Italy under the fascist regime. Punk groups used very often the spray stencil reproduction technique to disseminate anti-establishment slogans. During the late seventies the Crass, initiated a great stencil anti-establishment campaign at the underground stations of London (Picture 15) that was diffused in such an extent, that later in the nineties when personal computers were commercialised, a type font for text editors with the authentic crass stencil style was named after the band's name [START_REF] Manco | History of Stencil Graffiti Stencil Graffiti[END_REF][START_REF] Berger | The story of Crass[END_REF][START_REF] Tsangaris | Radical Communication and Social Media-political slogans and street art in the time of the crisis[END_REF]. In this sense, an aesthetic counter cultural form was recuperated.

Some years later, during the nineties many graffiti writers turned to professional muralists using the form of graffiti-stencil for advertising purposes (Picture 16). Forms, processes and techniques that promoted free speech, alternative opinions and social contention lost their actual radical signification and got neutralized by the spectacle. To sum up, with the assistance of the typical two-sided semiotic saussurean sign, the signified as time passes undergoes an irreversible transformation through the process of recuperation. The initial bond between the signifier and the signified of counter cultural symbols that could threaten the capitalist system is distorted. Signifiers of counter cultural emblems are becoming common by the mechanisms of mainstream cultural industry and can be related also to fashionable, clothing, trendy practices etc.

Picture 17. Saussure's two-sided sign Picture 18. Saussure's two-sided sign during the process of recuperation

Recuperation and Social Media

The 'Spectacle' according the SI does not represent just images of fashion, advertisements, cinema and the media, it mostly consists a schema that describes how power operates non-coercively, not from a single authority but from several superficially unrelated, unprejudiced sources to achieve a homogenous effect (Crary 1989). The spectacle appears always under a variety of different trends, various ideologies, contrary life styles, etc. It urges people to choose just one of these main sources in order not to question the totality.

For [START_REF] Downing | Radical Media[END_REF], the expression 'alternative media' is an oxymoron. The term can signify all different suggestions that each of them has the possibility to take the place of the main system and become conventional. That is, by defining mainstream corporations as the 'standard' condition for media, a great number of alternative versions arise in relation to their content, organization, ownership, technology, location, etc. such as oppositional media, activist media, citizens media, tactical media, radical media, social movement media, community media and so on. [START_REF] Atton | [END_REF], made a point arguing that alternative media are crucially about offering the means for democratic communication to people that are excluded from media production. In this respect, [START_REF] Fuchs | Alternative media as critical media[END_REF] considered that alternative media studies are often strongly connected to anti-authoritarian perspectives due to their shared focus on selforganizing practices that tend to idealize small-scale production.

In this study I located specific examples of alternative media counter cultural forms, recuperated by the dominant culture mechanisms, that can be easily diffused by social media. Nowadays, images of counter cultural emblems are promoted extensively by social media, sometimes neutralizing their initial antiestablishment qualities. Perhaps, we are marching through a final stage of capitalism through a complicated process that can be defined as the phase of social media recuperation.

Social media assemble integrated tools of communication and cooperation within the commercialized web (or world) that can be used by anyone. The social media have changed the way typical mediated communications work. The conventional media ecology was disrupted irreversibly as radical media could be embedded to mainstream communication platforms.

There is a strange connection between counter cultural perspectives and social media. This stage interrelation is initially reflected by the permanent ambiguity regarding sovereignty and the internet structure that fluctuates between the concepts of centralised control capitalism and decentralized network democracy.

The influence of social media in contemporary society is an argument that has been bothering intensively for the last decades both the academic community and the world of politics. Hence, centralised platforms such as Facebook, YouTube, Twitter, Instagram, Tumblr and so on, have been assumed to be in effect very important for the processes of individual and collective identities construction, for the genesis and maintenance of virtual communities as well as for the coordination of social movements. Undoubtably, innovations and new technologies always improve communication between people. Social media platforms offer advanced opportunities for multicultural international communications between local radical activists setting a new framework for united international struggles that need common communication channels for global motivation against capitalism.

Contemporary information and political studies aroused questions about the possibilities of the networks to produce collective and connective action. Accordingly, new network technologies, may (or may not) determinedly lead to social action without necessarily requiring the preexistence of a collective identity (Bennett and Segerberg 2013[START_REF] Castells | Networks of Outrage and Hope[END_REF][START_REF] Gerbaudo | In search of the 'we'of social media activism: introduction to the special issue on social media and protest identities[END_REF][START_REF] González-Bailón | Networked discontent: The anatomy of protest campaigns in social media[END_REF].

One way or the other, the new media were broadly used by citizens in protest organization as free channels, transmitting messages and images without censoring by the states. However, the realization of the protests always took place in the streets with the physical interaction between people. After all, the concrete movements were actualized by people in the streets of Cairo (2011), Madrid (2011), Athens ( 2011), New York (2011) and many other places, where individual activists met and interacted face to face [START_REF] Hardt | [END_REF].

Additionally, someone could easily claim that social media have initiated a recuperation process that rearranges counter cultural emblems and anti-establishment slogans to appear like commercial advertising or comic posters.

Unfortunately, their user generated content in combination to their commercial orientation assemble heterogeneous portfolios that store up together the comic with the disastrous, the significant with the unimportant, the real with the fake, the moral with the corrupt, the public with the domestic, the political with the commercial and so on, bringing everything to an 'eternal, illusory party' [START_REF] Ludovico | Face-to-Facebook, Smiling in the Eternal Party[END_REF].

This condition certainly does not recall decentralized and secure communications systems that increase the opportunities for citizens to take part in public affairs. On the contrary, it constructs a mechanism that increases recuperation and apathy. As [START_REF] Morozov | To save everything, click here: The folly of technological solutionism[END_REF] argues, it rather brings about passive states of participation such as 'solutionism', 'slacktivism' and 'armchair activism'.

Counter cultural images are posted to social media by a great number of individuals, with different aims that are connected to discrete social groups. This means that people with different directions, that are involved in diverse 'language-games' [START_REF] Wittgenstein | Philosophical Investigations[END_REF] can use the same images to signify diverse concepts.

The meanings of the images in social media are flexible and can take many connotations depending on the profile and the social capital of the individual that makes them available publicly on the platform. In this sense, although nowadays there is a great diffusion of counter cultural visual emblems through social media, those emblems are loaded with multiple meanings and this fact finally neutralizes the original purpose of the prime symbolic connotation.

Eventually, it must be clear that social media are not public services or open communal spaces designed to advance social discourse, supporting civil society [START_REF] Lovink | Unlike us reader: Social media monopolies and their alternatives No 8 -Institute of Network Cultures[END_REF][START_REF] Seemann | Das neue Spiel: Strategien für die Welt nach dem digitalen Kontrollverlust[END_REF]. Social media do have embedded commercial politics as private corporations of the 'platform capitalism' [START_REF] Srnicek | Platform capitalism[END_REF] and unquestionably their basic objective is the sustenance of 'spectacle capitalism'.

Thus, in present-day terms social media target the endless accumulation of data by getting as many 'prosumers' [START_REF] Toffler | Powershift: Knowledge, Wealth, and Violence at the Edge of the 21st Century[END_REF] or 'produsers' [START_REF] Bruns | Blogs, Wikipedia, Second Life, and Beyond: From Production to Produsage[END_REF] as possible, supporting once more the commercialised economy as the only basis of the human civilization.

Conclusions

In order to comprehend the consequences of the 'empire of illusion' [START_REF] Hedges | Empire of illusion: The end of literacy and the triumph of spectacle[END_REF] it requires a sufficient level of visual education. Visual critical pedagogy recognizes, analyzes and critiques how economic, political and social realities define and shape our perception of the world.

Additionally, visual education enhances critical consciousness and challenges students to analyze how values beliefs and knowledge are interrelated with issues of agency, politics and power [START_REF] Tavin | Wrestling with angels, searching for ghosts: Toward a critical pedagogy of visual culture[END_REF].

Thus, if the investigation of counter cultural emblems constitutes an effective pedagogical tool for achieving emancipatory knowledge for social transformation, it should be broadly recognized as more than a way to explain the behaviour of counter cultural groups.

It has been said that the counter cultures perform an important task on behalf of the dominant culture, making it more flexible, through all these processes of resistance and recuperation. Art revolutions, counter cultural emblems or alternative life-styles never brought the system to its knees, on the other hand they always assisted for the revitalisation of the consumption (Clarke et al 2000: 65).

The dominant capitalist culture represents and reproduces itself continuously. In order to be updated, it defines, categorizes and intergrades all other cultures within its range. Mainstream culture stands as the most natural and universal worldview system, and this fact became even broader in the time of the social media. All other cultures must struggle to survive within it, to negotiate, to convive, to resist and in most cases to be assimilated.

Symbols of resistance after they become myths related to the values of counter cultures that represent are gradually removed from alternative media to mainstream channels.

Just as the original punk subculture was semantically neutralised and commodified by the culture industry, in the same way counter cultural processes and rituals such as tattooing, and graffiti have been invalidated and repackaged as new popular trends and fashion.

There is no doubt that trends and fashion are intertextual systems that sometimes may link products with counter cultural emblems. However, this process neutralises the original meaning of the emblems.

Commodification brings everything into the context of commodity-form, in order to be exploited as a new source of investment.

On the other hand, recent research has shown that sometimes counter cultural emblems can coexist as symbols political resistance despite the commoditization without losing their political power.

Marketing managers realised that it is very easy to motivate the consumers by activating their covert sentiments of indignation and rebellion, using counter cultural emblems and codes of the apparent enemies of massive consumption.

Previously confrontational visual symbolisms such as spiked and mohawk hair, leather accessories, ripped clothing, tattoos and graffiti, instead of motivating offensive rebellion and transgression have been transformed to commodity codes signifying the stylish, the exciting and the spectacular, within the frameworks of conformity.

Digital technology and the new media communications intensify this process by making universally apparent the counter cultural emblems and by promoting further resistance as commodity, introducing everything to the exchange-value system in relation with all the other commodities. This process reduces the signified concept of rebellion to the act of consumerism.

The social media gave extensive potentialities to everyone for the recuperation process. Images of counter cultural emblems and rituals can be posted by anyone, they can get organised in groups, become marketable in t-shirts and stickers and select 'likes', totalizing the commodity culture by elevating the accumulation of digital databases.

Finally, depending on the profile and the social capital of the individual that makes images publicly available on the social media platforms, meanings can take various connotations reflecting each time the rhetoric of the addresser.

'The most modern aspect of the spectacle is also at bottom the most archaic' [START_REF] Debord | Society of the Spectacle[END_REF].

Representations in social media, counter cultural emblems and alternative mechanisms of communication are all subjected to 'reification' and 'recuperation' [START_REF] Lukács | History and class consciousness[END_REF], Debord 1969), mixing radical struggles together with other commercial practices that belong to the sphere of 'Spectacle'. '

The spectacle is the self-portrait of power in the age of power's totalitarian rule over the conditions of existence' [START_REF] Debord | Society of the Spectacle[END_REF]. Critical investigation of visual elements that are published in social media can release students from what the Situationists International considered as the passive culture of spectacle, towards more active 'ways of seeing' [START_REF] Berger | Ways of seeing[END_REF]) and bring about possibilities for dynamic social engagements.

Picture 1 .

 1 Manchester punks 1976 Picture 2. Top ten haircuts 2016At that times an image of three young males dressed in typical punk clothing, gesturing ironically at the camera (Picture 1) denoted concepts such as rebellion, hooliganism and anarchy. If this photograph was published in the daily press, it would possible provoke disapproval, anger or disgust of the bourgeois society. This image represented an alarm and a possible threat for the conservative middle-class of the seventies.

Picture 7 .

 7 Tattoo: Les Skuse, Bristol, Great Britain, 1960s Picture 8. Live Unbuttoned Levis 501, Josh Beech photographed by David Vasiljevic

Picture 9 .

 9 Skinhead tattoo Picture 10. Lego tattoo campaign

Picture 15 .

 15 Crass graffiti at Bond street station 1979 (Dial House Collection) Picture 16. Living in Harmony -Coca Cola, TATS CRU, Inc

This paper has been presented at the XIX ISA World Congress of Sociology / RC57 Visual Sociology, Toronto Canada, 19 July

'Cooption-Cooptation is the process of absorbing new elements into the leadership or policy, determining the structure of an organization as a means of averting threats to its stability or existence. This is a defensive mechanism, formulated as one of a number of possible predicates available for the interpretation of organizational behavior'[START_REF] Selznick | Foundations of the theory of organization[END_REF]

34). Many scholars use the term cooptation in the same sense as I use the term recuperation in this article[START_REF] Hebdige | Subculture: The meaning of style[END_REF] Potter 2007, Luedicke and[START_REF] Luedicke | Towards a narratology of brands[END_REF]

The punk music scene although evolved as a do-it-yourself counter culture, it kept a very strange relationship with the commercial world and the fashion industry. As early as 1978,Laing (1978:127) had observed that punk clothing, 'was soon transformed by the avant-garde of the fashion industry into a new and expensive trend'. Dave and Stuart Wise the core of the 1960's British revolutionary group King Mob, once part of the English section of the SI, argued that although punk was commercialized, becoming a form of recuperation that turned the anti-authority movement into aggressive pop music, at the same time some people that were attracted to punk were transcending that representation. Thus, there was a lot of punk expression in the riots of 1991 in Great Britain which was direct struggle on the street. The people that were attracted to punk managed to brake their initial representations, and, in this sense, recuperation can break down when something real is happening (Permanent Culture Now, 2016) 'Veteran Situationists behind King Mob and 'Revolt Against Plenty', YouTube (29/4/2018) https://www.youtube.com/watch?v=ttSCSpnDSqA)

The process of 'détournement' is a guerrilla communication practice initiated by the Letterist International and the SI referring to reformed artistic representations and techniques that can turn the expressions of capitalism and its culture against itself[START_REF] Debord | Mode d'emploi du détournement[END_REF]

This work has been partly supported by the


Cyrille Hamon
email: hamon.cyrille@gmail.com

Luis M Liz-Marzán
email: llizmarzan@cicbiomagune.es

Colloidal Design of Plasmonic Sensors Based on Surface Enhanced Raman Scattering

Keywords: Surface enhanced Raman scattering, SERS, colloidal synthesis, plasmonics

This feature article focuses on the use of colloid chemistry to engineer metallic nanostructures toward application in surface enhanced Raman scattering (SERS) sensing, in particular for 'real-life' applications, where the analyte may be present in complex mixtures. We present a broad summary of the field, including recent advances that have been developed during the past 10 years. Real-life applications require a rational design and we aimed at identifying the key elements involved in it. The discussion is centered around colloidal plasmonic nanoparticles and therefore we start from the library of morphologies that have been reported in the literature.

To complete the picture, colloidal self-assembly, surface chemistry and the combination with materials science techniques are highlighted. Considering the progress in the field, SERS may ultimately realize its full potential as an ultrasensitive tool for routine analytical applications.

Introduction

Chemical sensors can be defined as devices that transform a chemical signal into analytically useful information. A chemical sensor is essentially composed of a molecular recognition element, the receptor, and a transducer that can convert a binding recognition event into a measurable signal. This definition is very general and encompasses mostly all types of chemical sensors. However, selecting a transducer or a receptor among the vast collection that has been reported in the literature is not an easy task. In this context, previous reviews classified sensors according to their figures of merit, such as sensitivity, selectivity, reproducibility, high throughput and potential use by non-experts. Electrochemical sensors, ion channel switches, fluorophores in e.g. enzyme based assays or else piezoelectric based transducers, have been critically reviewed in this way [1][2][3]. We aim herein at applying similar concepts to surface enhanced Raman scattering (SERS) spectroscopy, as an emerging analytical tool. SERS was discovered in the 1970's [4,5], but ever since the field appears to be in constant rejuvenation, partly due to the great progress in the development of well-defined colloidal plasmonic nanoparticles, which act as the transducer in a SERS-based device. A few but important points must be considered to capture the essential features of this technique. First of all, the transduction mechanism originates from the plasmonic properties of metallic nanoparticles that can confine light into nm-sized areas that feature intense electromagnetic fields, so-called hotspots, where SERS enhancement preferentially occurs. Therefore, the analyte should be localized as close as possible to the nanoparticle surface, typically at distances below 10 nm, so that SERS can be measured, rather than conventional Raman scattering.

Second, the optical read-out corresponds to the vibrational fingerprint of the analyte. Although both related to vibrational transitions, Raman scattering is significantly distinct to IR spectroscopy and we find it is worth spending a few words to explain the main differences. A first discordance can be found in the physical mechanism, as Raman scattering is based on inelastic scattering while IR spectroscopy is based on IR light absorption. It should also be stressed that, the information on vibrational transitions provided by both techniques is complementary, strong signals in IR spectroscopy usually being weak in Raman scattering spectroscopy and vice versa. A particularly relevant example is the strong water signal in IR spectroscopy, which is weak in Raman scattering, thereby providing us with the opportunity to readily investigate aqueous and biological samples. Interestingly, plasmonic effects can be used in IR spectroscopy, leading to so-called surface enhanced infrared absorption (SEIRA) spectroscopy, but significantly less effort has been put into this direction so far, presumably because of the poorer limit of detection (LOD), as compared to SERS. As we focus this review on SERS, we refer those readers who are interested in SEIRA, to other specialized reviews [6,[START_REF] Griffiths | Spectroscopic Properties of Inorganic and Organometallic Compounds: Techniques, Materials and Applications[END_REF]. Overall, SERS is promising as an analytical technique, as it provides molecule-specific information with enough sensitivity to study either a single analyte or various species simultaneously, since molecular vibrations can be readily distinguished (SERS peaks are typically narrow). Although companies such as Ocean Optics, Silmeco or Hamamatsu among other, already sells SERS substrates made of metallic nanostructures, SERS is not yet widely used as an analytical technique mainly because it suffers from certain limitations, such as issues related to signal contamination and insufficient reproducibility. Note that commercially available SERS substrates are in general prepared by physical deposition techniques but not from wet chemistry approaches. On the other hand, SERS substrates made by colloidal synthesis approaches would results in atomically smooth, well-defined surfaces, while containing tailored nanoscale features, thereby improving the measurements reproducibility.

Herein, we thus restrict the discussion to colloidal approaches for SERS and provide the key elements that may help transfer new SERS substrates to the market. In this feature article, the spotlight is directed toward the importance of crossing fields such as colloid science, surface chemistry and materials science, to meet the main challenges associated to SERS detection. We schematically show our view on this scope through a spider-web diagram, in Figure 1. The article is structured in such a way that it presents what we consider the 5 most important points that should be considered toward addressing the major issues associated to SERS (Figure 1), namely reproducibility, simplicity, selectivity, high throughput and sensitivity. A suitable combination of these parameters should be used to optimize the overall sensing performance, as indicated by the spreading red area in Figure 1. By presenting the recent literature within this general framework, we aim not only at pinpointing diversity in SERS based technologies, but also to help building bridges between different communities, which might be key toward addressing current and future challenges.

Analytical Applications of SERS

The potential impact of SERS in various aspects of analytical chemistry is summarized in Table 1, where we collected representative examples from the literature, spanning a broad range of applications. We listed experimental parameters from selected literature reports, where different plasmonic substrates, surface chemistry and SERS setups were used for detection in complex media. A fair comparison of the SERS performance for the numerous works presented here is however not an easy task because of the wide variation in working conditions.

Concentration ranges are indicated in the table, rather than LOD, because this parameter is not consistently provided in all manuscripts. Concentration ranges indicated in Table 1 may appear occasionally too high, as compared to often reported ultralow detection limits, which illustrates the difficulties to be expected when analyzing an analyte in real samples, rather than under laboratory, contaminant-free conditions. Although the information contained in each column of the table might deserve a separate review, we intend to present an overall picture of the field.

In the following sections, the experimental conditions used in each example are detailed, with a focus on the current strategies toward boosting the selected figures of merit of SERS sensors (Figure 1). After a short description of the plasmonic features required for efficient SERS (Section 2), we describe methods to improve signal reproducibility through colloidal selfassembly (Section 3). We then discuss how the nanoparticle surface chemistry can affect the selectivity toward a specific analyte (Section 4), and we close the discussion by presenting recent advances in the use of various aspects of materials science toward SERS detection (Section 5).

Plasmonic Nanoparticles as Transducers

The intensity of the measured Raman signal is closely related to the intensity of the electromagnetic field acting on the molecule, which can be largely enhanced when the molecule is adsorbed on the surface of plasmonic nanoparticles, which therefore become an essential feature of SERS. The so-called electromagnetic enhancement in SERS originates from localized surface plasmon resonances (LSPR), occurring when the conduction electrons in a metal nanoparticle collectively oscillate, in resonance with the frequency of incident light. Therefore, the analyte molecule not only experiences the electromagnetic field of the laser beam, but it also interacts with the field induced by the LSPR oscillation, which is primarily concentrated at vertices, tips or edges of metal nanoparticles. Hybridization of plasmon modes (also called plasmon coupling) originates when nanoparticles are located at very short distances, which can result in 2-3 orders of magnitude stronger electromagnetic fields at interparticle gaps, as compared to individual nanoparticles [21][22][23]. Such localized regions of intense local field enhancement are usually termed hot spots. Even though the electromagnetic enhancement mechanism is a central element in SERS, chemical enhancement originating from specific electronic interactions, such as charge transfer between the analyte and the plasmonic transducer, may also play a role [24]. Since the first observation of the SERS effect at rough silver electrodes, a wide variety of plasmonic substrates with SERS enhancing activity have been reported, which can be prepared by either top down (e.g. lithography) or bottom up (e.g.

self-assembly, colloidal synthesis) approaches [25]. We restrict the discussion here to the sufficiently wide scope of colloidal methods, in which nanoparticles are produced by wet chemistry approaches.

In practice, a close match between the laser energy and LSPRs at the nanoparticles is required to obtain a sufficiently high signal. In this regard, nanoparticles made of silver and gold (Au/Ag NPs) are the most frequently used transducers for SERS, because their LSPR frequencies can be tuned within the visible and near-IR (NIR), thereby matching commercially available laser sources. It should be noted that a significant body of research is currently in progress [26][27][28][29] to replace coinage metals, but few alternative materials have been identified so far, and their enhancement properties in the visible and NIR ranges are still limited [30]. The plasmonic properties of colloidal spherical Au NPs are typically characterized by a deep red color, which can however be modified by tuning particle size, between 3 -200 nm, leading to LSPR maxima ranging from 515 up to 750 nm [31,32]. For large particles (above 100nm) a dephasing effect is observed that lead to higher order plasmon modes and therefore new LSPR peaks. This influences the SERS intensity that have been observed to increase from 0 to 100 nm and then to decrease due to the increasing significance of radiative damping [33,34]. Another study found similar results but pointed out that the NPs were not fully comparable in terms of sphericity and crystallinity as the larger particles exhibit higher facets size and tends to be more anisotropic [35]. Moreover, the SERS performance depends on the match between the LSPR position and the excitation sources, thus the relation between the size of NPs and the SERS intensity is hard to disentangled as the LSPR is shifted for the different NPs batches. In the case of small spherical Ag NPs, the LSPR occurs at higher energies around 400 nm, but can also be tuned by increasing particle size [36]. Similarly to Au NPs, the relation between the Ag NPs size and SERS signal intensity have been investigated and particles diameters around 55-70 nm gave the best performances for excitation wavelengths at 532, 633 and 785 nm [34]. Nano-alloys made of gold and silver have also been prepared by co-reduction of the corresponding metallic salts in solution, thereby shifting the plasmon resonances within the range between 380 and 520 nm [37]. For applications that require a wider spectral tunability, alternative strategies have been developed to control not only the size but also the shape of the nanoparticles. For instance, LSPRs can be considerably red-shifted by increasing the anisotropy (aspect ratio) of gold nanorods, even reaching the IR range. In a recent example, concepts borrowed from controlled living polymerization were successfully applied to the synthesis of bimetallic long Au@Ag nanowires, which can accommodate multiple plasmon resonance modes, spanning from the visible up to the mid-IR [38]. In this work, the authors showed that the silver dominated the optical properties in the near field with the gold core playing a marginal role. The selection of gold or silver nanoparticles for SERS is usually tailored to the targeted application, as both metals display excellent performance, albeit with some differences. A major parameter to be considered is the chemical stability, which is significantly higher in gold, thereby affecting the interaction between the NP surface and the analyte molecules. On the other hand, silver often yields higher enhancement factors, in particular at low wavelengths. The better plasmonic performance of silver is related to the metal interband transitions being restricted to the UV range, so that they do not interfere with plasmon resonances. Note that protecting the silver nanoparticle with a gold layer is possible in order to combine the high optical properties of silver particles with an improved chemical stability [39][40][41]. Practical aspects must also be considered, as the synthesis of gold nanoparticles has been described in more detail, so that a wider library of particle sizes and morphologies are available. Although spherical NPs have been most widely studied for SERS, anisotropic NPs possessing sharp tips and edges are known to display higher near field and SERS enhancements [42]. The selection of NP morphologies for SERS typically relies on the corresponding LSPR wavelength and field enhancement distribution. Recent development of electron microscopy methods has opened the possibility to characterize hot spots in situ, on both single particles and assemblies, by measuring either electron energy loss spectroscopy (EELS), or emitted photons having energy in the visible, by cathodoluminescence [43,44]. Since the first application of EELS to map surface plasmon resonances on silver nanotriangles [45], a wide variety of shapes have been investigated, as exemplified in Figure 2. of plasmonic nanoparticles and corresponding EELS maps (lower panel). Colors are arbitrary, but brightness indicates areas of higher field enhancement. A) Gold nanotriangle [46]; B) Silver nanorod [47]; C) Gold nanostar [48]. Scale bars on all images are 50 nm. A), B) and C) were adapted with permission from references [40], [41] and [42], respectively. Copyright American Chemical Society.

Since NP shape determines light confinement at the nanoscale, it also makes a strong impact in SERS applications, but it is difficult to conclude which NP type prevails. It can be seen in Figure 2 that, only a small fraction of the NP surface displays high field enhancements, where the SERS effect is produced [49]. There seems to be general agreement that multi-branched nanoparticles, also known as nanostars, are the brightest nanoparticles reported so far; many works pinpointed their high performance for SERS [50][51][52]. As an example of practical application, the detection of the pesticide thiabendazole has been reported down to 20 ppb on fruit skin, using nanostars immobilized on a flexible and transparent polymer substrate [12].

Nanostars can be produced by means of seed-mediated growth methods, in which spherical nanoparticles are rapidly injected in a growth solution containing both metal salt and shapedirecting adjuvants [53,54]. Generally speaking, the synthesis of anisotropic nanoparticles requires a certain degree of control over the rate at which atoms are deposited on certain crystallographic facets of the seeds [55,56]. In the case of nanostars, the deposition is largely governed by reaction kinetics, resulting in a dispersion of branched nanoparticles with a rather wide variety of branch length and sharpness, even within individual particles. As a result of the low thermodynamic stability, nanostars can spontaneously reshape into spheroidal particles and therefore a suitable capping molecule or coating shell are required upon synthesis. Usual capping agents are poly(N-vinylpyrrolidone) (PVP), cetyltrimethylammonium bromide (CTAB) or thiolated molecules, while coating with silica has also been demonstrated.

Unfortunately, the coating process may hinder adsorption of analyte molecules on the metal surface, in turn compromising SERS performance. Thus, a balance must be found when working with nanostars, between long term colloidal stability and SERS performance, which can be solved by cleaning the surface (removing capping ligands) prior to the analysis. As an alternative, other plasmonic transducers can be used, which may not be as efficient toward electromagnetic enhancement but display much better long-term stability, size monodispersity and ease of synthesis. Another popular choice are gold nanotriangles, which are significantly more stable against reshaping, with SERS enhancement factors in solution (from individual particles) up to 10 5 [57]. Even though the synthetic protocols usually result in moderate shape yield, purification can be successfully applied to remove byproducts (particles with other morphologies), so that the content of nanotriangles can be increased above 90%.

Other nanoparticle morphologies have been studied for SERS applications, such as nanobipyramids [49], porous nanoparticles [58][59][60] and nanoparticles featuring internal nanometer-scale gaps [61][62][63][64][65][66], though practical applications are yet to be demonstrated. More frequently used are gold nanorods, which can be readily produced in high yield (>90%) and a variety of protocols have been developed for their preparation, with fine tuning of both particle size and aspect ratio, by simply varying a few synthesis parameters [67][68][69]. However, their SERS performance at the single nanoparticle level is inferior to that from nanoparticles with sharper tips. A detailed theoretical study has been recently reported, aiming at a fair comparison between the three most frequently used NP shapes for SERS, i.e. nanospheres, nanorods and nanostars [70]. The computational results concluded that nanostars provide the best performance at the single particle level, but nanorods outperform other shapes when assembled in dense monolayers. For further reading, a detailed analysis on the plasmonic properties of anisotropic nanoparticles can be found in a recent review by our group [42]. This discussion serves us to introduce the following section, which focuses on hot spot engineering, as a result of collective plasmonic effects.

Improving SERS Detection Through Self-Assembly

Despite of the wide variety of nanoparticles reported so far, gold and silver nanospheres are still most frequently used toward 'real-life' SERS applications (see Table 1), which stems from a trade-off toward simplicity at the expense of sensitivity. Although plasmonic nanospheres are not particularly SERS efficient at the single particle level, their performance can be greatly increased within assemblies. In practice, the aggregation of nanospheres can be readily induced by addition of charge screening agents (e.g. electrolytes), so that analyte molecules can be trapped at the resulting hotspots within the aggregates. By means of this simple process, the drug levofloxacin has been quantified in urine samples at concentrations down to 4.5.10 -4 M which is highly relevant to monitor clearance rate. In certain cases, the analyte alone can induce the aggregation of the nanoparticles, which has been demonstrated for the volatile compounds 4-aminobenzenethiol and 2,4-dinitrotoluene, which can be detected from the gas phase at trace levels (1 ppb). However, uncontrolled aggregation often leads to a wide distribution of areas with different enhancement factors, thereby hindering quantification of the SERS signals.

Within this framework, a vast amount of work has been carried out to improve signal reliability by directed self-assembly strategies (Figure 3) [71][72][73][74][75][76][77]. SEM images of a gold nanorod supercrystal, viewed from the top and from the edge [9]. D) SERS detection of scrambled prions at various concentrations in bovine serum, using gold nanorod supercrystals as the transducer [9]. A,B were adapted with permission from reference [66]. C,D were adapted with permission from reference [9]. Copyright 2011 National Academy of Sciences.

Self-assembly in solution or at interfaces (i.e. solid/liquid, liquid/liquid) are mainstream approaches toward the preparation of SERS substrates. This is represented on Table 1 wherein the nanoparticles are either in a solid-state material or dispersed in colloidal suspension. The design of the SERS transducer is related to the goal of the study. For instance, a solid form can be used in harsh conditions (e.g. high ionic strength) whereas a colloidal form is particularly suited to probe various region of a sample (e.g. cells internalization). Colloidal nanoparticle clusters with controlled size (aggregation number) can be prepared by means of association with block copolymers [78] or DNA origami [79][80][81], among other approaches [64,[82][83][84][85][86][87]]. An appealing feature of colloidal plasmonic structures is the opportunity to interact with their environment, whereby the building blocks can be encoded with SERS tags [88,89], but also associated with luminescent or magnetic nanoparticles, leading to multi-functional clusters [83,[90][91][92]. For instance, the co-encapsulation of gold nanostars and iron-oxide nanoparticles resulted in the formation of hybrid colloidal composites that were highly responsive to both light and an external magnetic field [91]. Limits of detection around 10 -9 M for model analytes were demonstrated upon magnetic accumulation, while the relatively small size of the clusters renders them an appealing system for bioimaging applications. On the other hand, the size of the plasmonic structure is no longer a limit when immobilized at an interface, i.e. the density of hot-spots can be dramatically increased, even in three dimensions. Monodisperse building blocks such as polyhedral [93,94] and anisotropic nanoparticles [95], are required to achieve uniform nanostructures, which usually show excellent SERS performance. In line with this, arrays of standing gold nanorods have been shown to be ideal substrates, as they display a homogeneous distribution of hotspots with very high density, thereby ensuring both reproducibility and sensitivity [96]. Highly uniform SERS signals were measured by scanning the surface of organized nanorod supercrystals, and enhancement factors as high as 10 7 were obtained [97]. Self-assembled gold nanorods have also been used for the detection of the pesticide carbaryl in the ppm regime, in apple juice [START_REF] Zhang | [END_REF]. In another example, scrambled prions dispersed in bovine serum were detected down to 0.1 nM concentrations, even from whole blood, thus showing great promise for early diagnosis of neurodegenerative diseases [9]. The various limits of detection reported herein are not only influenced by the chemical nature of the analyte, but also by the complexity of the surrounding medium, which often requires tailored surface chemistry.

The Relevance of Surface Chemistry

The spotlight of this review has been so far directed toward the metal core of plasmonic nanoparticles. This section however focuses on their surface chemistry, which can be equally important when dealing with SERS applications. First of all, the molecular structure of the analyte must be taken into consideration at the time of selecting a SERS substrate. On one hand, detection can be optimized if an electronic absorption band of the analyte coincides with the energy of the exciting laser light, resulting in resonance Raman scattering, which in the presence of a plasmonic transducer leads to surface enhanced resonance Raman scattering (SERRS) [98].

On the other hand, the Raman cross section of the analyte is another intrinsic feature to be considered [99]. In general, heterocyclic molecules are characterized with the highest Raman cross sections and outperform other molecules like water or fatty acids. Considering the latter, it is not surprising that pesticides [100], various drugs [101], or dye pigments [102], which feature aromatic rings, are usual analytes of choice for SERS demonstrations. It is however more challenging to measure analytes featuring small Raman cross sections or those which do not even display relevant vibrations (e.g. metal ions), for which specific strategies have been devised [103]. In the example of ions, detection can be achieved by indirect SERS, i.e. using a Raman active ligand that can undergo a certain vibrational change upon exposure to the analyte [104]. This strategy has been used to quantify few ppb of mercury [105] and picomolar concentrations of chloride [106], in fresh aqueous samples (Figure 4a). However, demonstration in 'real-life' water samples was not shown, presumably because the presence of competitive atomic species would require the use of more specific molecular receptors. An alternative method consists of encoding nanoparticles with a Raman reporter molecule (SERS tag), as well as a biocompatible coating that can be functionalized toward a specific receptor [107,108]. This approach may be used to detect analytes with low Raman cross section, contamination issues from the transducer are overcome, and multiplexing is possible by the simultaneous use of different SERS tags, so that various analytes can be targeted for spectroscopic biological detection or imaging applications [88,[109][110][111][112][113][114]. SERS tags that do not carry a protective layer are capable of probing both the position of the nanoparticle (imaging) and the local environment [114]. For instance, pH has been measured by SERS within living cells, employing a pH sensitive Raman reporter, which is highly relevant to probe pH in the different cell compartments (Figure 4b) [15,115]. A second type of SERS tags features a protective coating, such as polyethylene glycol or inorganic shells [107]. Whereas the coating hinders interaction of SERS reporters with their surroundings, it can enhance cellular uptake and colloidal stability, thereby making them particularly suited for imaging purposes. In a representative example, five different silica-coated SERS tags were spectrally separated and located in vivo after accumulation in the liver of a mouse [110]. Further discussion on applications of SERS tags in bionanalysis is beyond the scope of this feature article and we direct the reader to recent reviews [88,107]. [11]. A,B were adapted with permission from references [91] and [15], respectively. Copyright American Chemical Society.

C was adapted with permission from reference [11].

The surface chemistry of nanoparticles may be modified to improve selectivity and facilitate ultradetection, in particular when the analyte is present at very low concentration in a medium containing other molecular species. In practice, there is a competition for space between the surface ligands, the analyte and potential contaminants, which must be balanced and generally results in a bottleneck when aiming at detecting very low analyte concentrations. Adsorption of the analyte on the plasmonic transducer is crucial and various issues must be addressed in this respect. It has been reported that SERS performance is enhanced when electrostatic repulsion forces between plasmonic particles and analyte molecules are minimized [116,117]. Similarly, hydrophobic/hydrophilic interactions or the molecular affinity between the analyte and the metal surface must be considered. For instance, the SERS signal of cytochrome C was found to depend on the charge of the plasmonic substrate, such that positively or negatively charged surfaces lead to different protein orientations [118]. Therefore, a compromise between avoiding contamination and promoting the diffusion of the analyte toward the plasmonic surface should be achieved. In this context, a recently reported strategy comprises the use of substrates made of gold nanorod supercrystals embedded in a mesoporous silica matrix, which acts as a molecular sieve that suppresses molecular diffusion of large (bio)molecules, while allowing the penetration of small analytes within the supercrystal [119]. Interestingly, it was demonstrated that such substrates can be applied to the detection of bacterial quorum sensing metabolites, at trace concentration level (below 10 -10 M), within a growing biofilm of Pseudomonas aeruginosa (Figure 4c) [11]. Even though other small molecules can also diffuse into the mesochannels, the chemical fingerprint of the analyte was clearly recorded in SERRS because pyocyanin displays an absorption band in the near-IR (resonant with a 785 nm laser), as well as intrinsically high Raman cross section. Other coatings can be used such as mesoporous titania, hydrogels or metal organic frameworks, which would additionally provide different means to tune the pore size [11,120,121]. Alternative approaches aim at avoiding biofouling by coating the nanoparticles with e.g. polyethyleneglycol or proteins. A recent report showed the hierarchical modification of the plasmonic transducer by a combination of a short probing ligand and a longer anti-fouling ligand, to detect doxorubicin spiked in whole blood at the micromolar range [122]. It should be noted that each application requires a different design and additional examples can be found in recent reviews focusing on the advances of SERS for in vivo diagnosis [107] and food safety [100].

Crossing Borders with Materials Science

Practical applications require a device that displays excellent performance, eventually allowing high throughput screening, while maintaining simplicity [123]. We describe in this section some opportunities that materials science can offer toward high throughput, reliability and eventually increased sensitivity. Elastomeric stamps prepared by soft lithography can be used in tandem with self-assembly to improve the deposition of nanoparticles on surfaces [124,125], as recently demonstrated through the deposition of gold nanorod arrays over centimeter length scales, which offers the opportunity to tune electromagnetic enhancement at interparticle gaps, with nanoscale precision [126]. Larger supercrystals can be obtained by means of this strategy, but using templates with features that are much larger than the dimension of the nanoparticles, resulting in identical multiple copies over large areas [127,128]. The resulting hierarchical organization was shown to improve signal reliability over the substrate, as compared to dropcasted substrates [127].

When dealing with SERS detection in 'real-life' samples, various treatments are usually needed prior the analysis, to remove interfering species or to concentrate the analytes, which hinder the widespread use of SERS. In this respect, materials science can help simplifying those steps, by using only benchtop equipment while saving processing time. As an example, superhydrophobic surfaces can be used to concentrate an evaporating aqueous dispersion directly onto the plasmonic substrate [129]. This concept has been further extended using so-called omniphobic surfaces, resulting in the development of a technique that is known as slippery liquid-infused porous surface-enhanced Raman scattering (SLIPSERS) [13]. These approaches present the advantage of directly concentrating the analyte within the sensor, thereby requiring minimum user intervention. The high collection efficiency of SLIPSERS led to LOD down to attomolar concentrations, for a model analyte dispersed in ethanol, and down to 1 ppb for polychlorinated biphenyls (PCBs) extracted from contaminated soils [13]. Other substrates such as paper can be combined with plasmonic nanoparticles toward flexible and low cost devices [130]. circulating cancer cells, pre-labeled with SERS tags [17]. A was adapted with permission from reference [13]; Copyright 2015 National Academy of Sciences. B was adapted with permission from reference [17]; Copyright 2015 American Chemical Society.

Microfluidics deals with the manipulation of small volumes into microchannels and holds great promise toward numerous applications, when combined with SERS [131,132]. Advantages include enhanced mass transport, low dosage and high throughput, as compared to conventional SERS systems. Care must be taken during the design of the sensor, such as the functionalization of the microchannel to avoid accumulation of nanoparticles within the SERS interrogation region [132], but properly designed systems have shown outstanding performance. For example, micromixers have been used to enhance the interaction between the analyte and the transducer, performing the detection of the pesticide Methyl Parathion in aqueous samples, down to 0.1 ppm [133]. On a different application field, circulating cancer cells at low concentrations were quantified by labeling with SERS tags and flowing them through a microfluidic chip, which opens new avenues toward continuous single-cell SERS analysis [17].

Similarly, bacterial strains have been identified with dramatically lowered analysis time, when SERS was combined with microfluidics, as compared to standard SERS (4h vs. 31h) [134].

Summary and Outlook

Although a large amount of research has been carried out in the field of SERS, ranging from nanoparticle synthesis through demonstrated sensing applications, the majority of this work has still been dedicated to proof-of-concept studies. During the last decade, 'real-life' analytical applications have also been demonstrated, which may result in a clear advantage, as compared to state of the art techniques. This feature article intended to demonstrate recent advances toward 'real-life' applications of SERS, by highlighting the need for a rational design in order to to improve sensitivity, reproducibility, selectivity, simplicity and high throughput of SERS-based devices. Overall, the advances presented in this review show that we are close to kickoff SERS into the real world and how this can be assisted by materials science technologies.

Some remaining limitations must however be resolved, before being able to purchase operating SERS kits commercially. Despite the eventual portability of microfluidic technologies, the SERS analysis of 'real-life' samples is usually carried out with bulky micro-Raman setups.

Demonstration of similar performance using portable Raman spectrometers should open new avenues toward applications requiring deployable systems. Moreover, microfluidics not only require a chip but also external equipment, which so far limits the applications of SERSmicrofluidic devices to the laboratory. To ultimately obtain a device that can be used by nonexperts, it is important to minimize the bulkiness and cost of the equipment as much as possible, while keeping the performance of the device intact. Considering these limitations, various strategies have been proposed to implement pump-free microfluidics, by means of pumping lids [135] or vacuum driven flows [136]. Such advances are available to simplify the use of SERS devices in the near future. An example of application is the separation of red blood cells from whole blood and the detection of biomarkers in "self-powered" chips [137]. We envision that similar concepts can be applied to SERS and make a large impact in the field.

Figure 1 .

 1 Figure 1. Spider web diagram in which the 5 edges correspond to one of the identified figures of merit of a SERS sensor. The red surface increases with the performance of the SERS sensor, by successively combining plasmonic nanoparticles, hot-spot engineering, surface chemistry

Figure 2 :

 2 Figure 2: Plasmonic nanoparticles concentrate light at their apexes. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images (upper panel)

Figure 3 :

 3 Figure 3: Self-assembly of plasmonic nanoparticles for SERS. A-B) Comparison of the SERS performance of colloidal clusters made of gold spheres; EF increases with coordination number [66]. B) Calculated near field intensity maps for four types of clusters, along the plane through the particle centers, showing intense plasmonic hot spots at interparticle gaps [ 66]. C)

Figure 4 :

 4 Figure 4: SERS studies adapting chemistry of transducer and receptor. A) Schematic representation of the detection of chloride by a Raman active ligand immobilized on silvercoated silica microbeads [91]. B) Probing and imaging pH inside individual live cells by a SERS nanosensor comprising a pH sensitive Raman reporter [15]. C) In situ detection and imaging of pyocyanin produced by Pseudomonas aeruginosa, grown on micropatterned gold nanorod supercrystals covered with mesoporous SiO2 (scale bars: 5 µm) [11]. A,B were adapted with

Figure 5 :

 5 Figure 5: SERS assisted by materials science. A) Working principle of SLIPSERS and detection of PCBs extracted from contaminated soils [13]. B) Microfluidic setup used to detect

Table 1 .

 1 Summary of experimental parameters used in the literature for SERS detection in real samples. Examples were selected on the basis of the complexity of the medium, so that the diversity of

	applications in SERS based technologies is represented. Details on acronyms can be found in the
	corresponding references. * Nanoparticles are in a solid-state material. ** Nanoparticles are in colloidal
	form.
	Plasmonic substrate	Coating	Analyte	Concentration range	Medium	Field of application	Ref.
	Au nanorod supercrystal*	CTAB	Carbaryl	5.10 -6 -1.5.10 -4 M	Apple juice	Food safety	[8]
	Au nanorod supercrystal*	CTAB	Prions	10 -10 -10 -6 M	Bovine serum	Food safety, disease prevention	[9]
	Au spheres*	Thin SiO2 shell	Methyl parathion	trace	Fruit skin	Food safety	[10]
	Au nanorod supercrystal*	Mesoporous SiO2	Pyocyanin	10 -14 -10 -4 M	Bacterial biofilm	Bacterial infections	[11]
	Au nanostar*	Citrate + SDS	Thiabendazole	10 -7 -10 -3 M	Fruit skin	Food safety	[12]
	Au aggregates*	Citrate	PCB 77	3.10 -9 M	Soil extract	Environmental monitoring	[13]
	AgNP 3D networks*	Glucose	Melamine	10 -6 -10 -3 M	Milk	Food safety	[14]
	Au aggregates**	Citrate + 4-MBA (pH sensitive)	pH	pH: 5.4-6.8	Intracellular environment	Diagnostics	[15]
	AgNPs**	Citrate + glucose	Pigments	-	Sample from art piece	Art preservation	[16]
	Ag clusters -SERS Tag**	PVP&BSA + SPDP (receptor)	Circulating cancer cells	-	Complex buffer	Cancer diagnostics	[17]
	Ag aggregates**	Citrate	Levofloxacin	10 -4 -10 -3 M	Treated urine	Drug monitoring	[18]
	Ag aggregates**	Citrate	Metham-phetamine	10 -9 -10 -3 M	Saliva (filtered)	Drug monitoring	[19]
	Ag NPs **	Citrate	4-ABT	-	Air	Environmental safety	[20]

Acknowledgements

This work was funded by the Spanish MINECO (grant # MAT2013-46101) and the European Research Council ((ERC Advanced Grant # 267867, Plasmaquo).