Shortcode attribute to display post from the last 24 hours

Hello,
I wonder if you could possibly help me. I have created a shortcode to display a custom post type on a yellow box within the pages of the site I am working on. I must warn you this is the fist time I write a shortcode so it’s not perfect.
What I would like to do is set up and attribute so that only posts published in the last 24 hours are displayed by this shortcode. Any posts older thatn 24 hours will just not come up. This is the code I have so far.

//[servicealert] - outputs a link
function service_alert($atts){
   extract(shortcode_atts(array(
      'posts' => 1,
   ), $atts));
};
function alertwd_shortcode() {
   query_posts(array('post_type' => 'alert', 'post_per_page' => '1' , 'showposts' => 1));
   if (have_posts()) :
      while (have_posts()) : the_post();
         $return_string = '<div class="alert">'.'<h3>'.get_the_title().'</h3>'.'<p>'.get_the_content().'</p>'.'</div>';
      endwhile;
   endif;
   wp_reset_query();
   return $return_string;
}
add_shortcode( 'servicealert', 'alertwd_shortcode' );
?>

Could you help?
Thank you!